# Class 10th Physics Unit 12 Question An Answer

## Class 10th Physics Unit 12 Question An Answer On Newsongoogle By Bilal Articles

Explore Class 10th Physics Unit 12 with detailed questions and expert answers on Newsongoogle by Bilal Articles. Elevate your understanding and excel in your studies. Unlock the secrets of Unit 12 now.

### Unit 12

Q 1. What is reflection?

Ans: Reflection of Light: When light traveling in a certain medium falls on the surface of another medium, a part of it turns back in the same medium. This is called the reflection of light. OR The bouncing back of light in the first medium after striking with any reflecting surface.

Q 2. Define regular reflection and Irregular reflection.

Ans: Regular Reflection: The reflection by a smooth surface in which all the reflected rays are parallel to each other is called regular reflection. Irregular Reflection: The reflection of light rays is not parallel to each other.

Q 3. Write the conditions for regular and irregular reflection.

Ans: Conditions for Regular Reflection:  The reflecting surface should be a plane surface.  The reflecting surface should be flat.  The reflected rays of light should be parallel to each other and equal-space.  Conditions for Irregular Reflection:  the reflecting surface should be rough.  The reflecting rays of light are not parallel to each other.

Q 4. What are the laws of reflection of Light?

Ans: A ray of light obeys the following laws of reflection:  The angle of incidence is equal to the angle of reflection.  the incident ray, the reflected ray, and the normal at the point of incidence, all lie in the same plane.

Q 5. Define the following terms: (i) Incident ray (ii) reflected ray (iii) normal (iv) angle of Incidence (v) angle of reflection (vi) Ray of light (vii) beam of light.

Ans: Incident Ray: The ray of light coming from a source of light is known as an incident ray. Reflected Ray: the light ray which is thrown back, when the incident ray hits the reflecting surface. Normal: A line (imaginary) at the right angle to the plane (surface) is called normal to the surface. The angle of incidence: the angle between the incident ray and the normal. Ray of light: the direction of path taken by light is known as ray of light Beam of light: a beam is a stream of light energy.

Q 6. What are the spherical mirrors? How light is reflected from spherical mirrors? Give the uses of spherical mirrors.

Ans: A Spherical mirror is a portion of the reflecting surface of a hollow sphere. Spherical mirrors are of two types:  Concave Mirror: The mirror whose inner curved surface is reflecting is called the concave mirror.  Convex Mirror: The mirror whose outer curved surface is reflecting is called the convex mirror.

Q 7. Write down the Uses of spherical mirrors.

Ans: nowadays spherical mirrors have a large number of scientific and practical uses. A few uses are given below: • Doctors use concave mirrors for examination of the ear, nose, throat, and eyes. • Concave mirrors with a parabolic shape are used in searchlights to throw an intense beam of light to a large distance. • Some people use a concave minor for shaving because when a man stands between the principal focus and pole of a concave mirror, he sees an enlarged, erect, and virtual image of his face. This is the reason why a concave mirror of a large focal length is used for shaving. • Concave mirrors are used to throw light on the slides of the microscope so that the slides can be viewed more clearly. • Nowadays developed countries use giant concave mirrors in their huge telescopes. • Convex mirrors are used in motorcycles and automobiles which enables the driver to see the automobiles coming behind him.

Q 8. Define the center of curvature, Radius of curvature, aperture, pole, and principal axis.

Ans: Center of Curvature: The center of the sphere, of which a concave mirror or convex mirror is a part is known as the Center of Curvature of the spherical mirror. Radius of Curvature: The radius of the sphere, of which a concave mirror or convex mirror is a part is known as the Radius of Curvature” of the spherical mirror. Aperture: The front section of a spherical mirror is circular and its diameter is known as the Aperture. Pole: The center of the concave or convex mirror is called the pole of the mirror. Principal Axis: A line joining the pole of the mirror and the center of curvature is called the “Principal Axis” of the mirror.

Q 9. Define the principal focus for concave and convex mirrors and why they are called the real and virtual focus.

Ans: Real Focus for Concave mirror: Rays of right parallel to the principal axis after reflection from a concave mirror converge to a point F. This point is called the “Principal focus” of the mirror. Since rays infect, and pass through this point, therefore, it is called real focus. Virtual Focus for Convex mirror: In the case of a convex mirror, rays parallel to the principal axis after reflection appear to come from a point F situated behind the mirror. This point is called the principal focus of the convex mirror. The principal focus of a convex minor is virtual because the reflected rays do not pass through it but appear to do so. Therefore, its focus is called virtual focus.

Q 10. Define focal length and write down its relation with the radius of curvature.

Ans: The distance between the pole and the principal focus of a spherical mirror (concave as well as convex) is called the “Focal Length”. It is denoted by f. Relation between Radius of curvature and focal length: The radius of curvature of a spherical minor is twice its focal length. i.e.

Q 11. Define linear magnification.

Ans: The ratio of the height of the image to that of the object is called linear magnification or simply magnification and is denoted by the letter m.

Q 12. Why convex mirrors are fixed on blind turns on the roads in hill areas?

Ans: Convex mirrors are fixed on blind turns on the road in hill areas because drivers from one side of the turn can see the automobiles coming from the other side of the mountain. So chances of accidents can be minimized.

Q 13. Why concave mirrors are used for the examination of the nose, ear, throat, and eye?

Ans: Concave mirrors are used for the examination of the nose, ear, throat, and eye to view a big and clear image of these organs.

Q 14. Why concave mirror is used for makeup or shaving?

Ans: Concave mirror is used for makeup or shaving because when a man/woman stands between the principal focus and pole of the concave mirror, he can see an enlarged image of his/her face. This is the reason why a concave mirror of a large focal length is used for shaving and make-up.

Q 15. Explain with the help of activity whether the image is smaller or larger, erect or inverted, real or virtual in a convex mirror.

Ans: Take a convex mirror or a well-polished spoon (using the outside of the spoon) and hold it in one hand and a pencil in the other hand with its tip in an upright position. Now look at the image in the mirror. The image is a convex mirror looks as • Appears smaller • Upright • Behind the mirror • Virtual Now an object moves closer to a convex mirror the image in a convex mirror looks as: • Moves closer to the mirror • Becomes larger • Upright • Stays virtual.

Q 16. Define mirror formula/ mirror equation. Also, write signs of convention.

Ans: Curved mirror formula is the relationship between object distance ‘p’ image distance ’q’ from the mirror and the focal length ‘f’ of the mirror. Mathematically it can be written as: 1 This equation is true for both types of concave and convex mirrors. But we have already seen that the image is formed sometimes in front of a curved mirror and sometimes behind it. This makes it necessary to have a sign’s convection so that we may distinguish between the two cases and obtain the correct answer when substituting in the formula. Explanation: showing the use of the two signs conventions in common use are given below: • All distances are measured from the pole of the mirror • Distances measured against the incident of light are positive. • Distances measured in the same direction as incident of light are positive.

Q 17. Explain with the help of activity whether the image is smaller or larger, erect or inverted in a concave mirror.

Ans: Take a concave mirror or a well-polished speed with a concave surface hold it in one hand and pencil it in the other hand with the tip in upright. The image formed by the concave mirror is • Real image • Inverted Bns: For all object positions outside the principal focus. But when the object is placed between focus and pole then the image is: • Virtual • Upright.

Q 18. What do you know about mirage?

Ans: In hot summer days, a reflection of motor cars is seen on the roads, and the image of an airo-plane is seen on the runway, infect there is no water on the runway. It is due to total internal reflection and this phenomenon is known as mirage.

Q 19. Write down the Law of Refraction.

Ans: The refraction of light takes place according to the following two laws: • The incident ray, the refracted ray, and the normal all lie in the same plane. • When a ray of light passes from one particular medium to another, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) 1s constant. This constant ratio is called the “Refractive Index” of the second medium concerning the first and it is denoted by the letter ‘n’. it can be mathematically written as: n = sin i sin r It is called Snell’s Law. A ray of light entering the second medium perpendicularly through the surface of separation shows no change of direction.

Q 20. What is Snell’s law?

Ans: When a ray of light passes from one particular medium to another, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant. This constant ratio is called the ‘Refractive Index’ of the second medium concerning the first and it is denoted by the letter ‘n’. it is called Snell’s Law. Mathematically can be written as: Sinθ1 It is called Snell’s Law. A ray of light entering the second medium perpendicularly through the surface of separation shows no change of direction.

Q 21. What is a refractive index? Write down the methods to calculate the refractive index.

Ans: When a ray of light passes from one particular medium to another, the ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is constant. This constant ratio is called the “Refractive Index” of the second medium concerning the first and it is denoted by the letter ‘n’. It can mathematically be written: Second method: the refractive index of a medium can also be calculated by dividing the speed of light in a vacuum by the speed of light in that medium. As the speed of light in a vacuum is almost equal to the speed of light in air. We use the speed of light in air instead of vacuum, while calculating the refractive index of a medium.

Q 22. Explain when the light changes its path, when pass through two different mediums.

Ans: During retraction, light bends towards normal as the light enters from a rate to a denser medium. • During retraction light bends away as light enters from the denser to rare medium. • The angel of incidence is greater than the angel of retraction. When light enters from a rate to a denser medium. • The angle of incidence is smaller than the angle of refraction when light enters from denser to rare medium. • if the light ray (incident ray) is along the normal (i.e. 90°) to the interface between the two mediums, the ray passes through without deviation. • No refraction takes place in the same medium.

Q 23. Define total internal reflection. What is meant by critical angle? Explain total internal reflection and the conditions necessary for it.

Ans: “When a ray of light front a denser medium enters a rarer medium in such a way that the angle of incidence is greater than the critical angle, then the ray is reflected inside and does not emerge out from the denser medium. This phenomenon is called total internal reflection”.

Q 24. What is meant by critical angle?

Ans: “The angle of incidence in the denser medium for which the corresponding angle of refraction is 90° in the rarer medium is called the critical angle. This angle of incidence is denoted by C.

Q 25. Write Conditions for total internal reflection.

Ans: The ray of light should travel from a denser medium to a rare medium. The angle of incidence should be greater than the critical angle.

Q 26. What should be the angle of incidence for total internal reflection?

Ans: The angle of incidence for total internal reflection should be greater than the critical angle.

Q 27. What is a prism?

Ans: Prism: Prism is a transparent body having three rectangular and two triangular surfaces. The angle of prism: the angle of the triangular surface opposite to its base is known as the ‘angle of Prism’.

Q 28. What is meant by the dispersion of Light?

Ans: The refraction of waves depends on their wavelength. Since the sunlight consists of different colors, the waves of different wavelengths, thus when it passes through a prism then the waves of different wavelengths deviate on different paths, due to this white light disperses in different colors, which is called dispersion. Solar spectrum: the band of colors that are seen after the dispersion of white light on the screen is called a solar spectrum.

Q 29. Define critical angle.

Ans: Critical Angle: Critical angle is the angle of incidences in the denser medium to which the corresponding angle of reflation in the rarer medium is 90°.

Q 30. What is a reflecting prism?

Ans: A reflecting prism has one of its angles equal to 90° and each of the remaining two angles equal to 45°.

Q 31. What is meant by Angle of Deviation?

Ans: The angle between the original path of the incident ray and the path of refracted rya through a prism is known as the angle of deviation. It is represented by D.

Q 32. Define lenses.

Ans: A transparent body has two surfaces of which at least one is curved. OR Lens is a piece of transparent medium bounded by two surfaces at least one of which is curved.

Q 33. How many types of lenses? Describe briefly.

Ans: Lenses are classified into two categories. 1) Convex or Converging Lens: The lens that converges all parallel incident rays after refraction is called a convex or converging lens. OR The lens is thicker at the middle and thinner at the edges. 2) Concave or Diverging Lens: “The lens which diverges the light at the point from all incident parallel rays.

Q 34. Define the power of the lens and its unit.

Ans: If the focal length of a lens is f then its reciprocal 1 f is called the power of the lens. Diopter: The unit of power “Diopter” is defined as the power of a lens with a focal length of one meter. Its symbol is ‘D’. Positive Power: Because the focal length of a convex lens is positive, therefore the power of a lens is also positive. Negative Power: Whereas the focal length of a concave lens is negative therefore the power of a concave lens is negative. Opticians and eye specialists refer to the power of a lens instead of its focal length for their patients.

Q 35. Which lens has greater power, the lens of less focal length or off greater focal length?

Ans: The power of a lens is given by the formula: p = l/f Where “f” is the focal length in meters. Hence for a lens of large power, the focal length will be small.

Q 36. To get a greater and clearer Image at what distance lens should be placed from the object?

Ans: To get a greater and clearer image, the object should be placed within the principal focus of the lens so that a clear, enlarged, and erected image is obtained.

Q 37. How image formation from a convex lens can be explained?

Ans: Image formation in a convex lens can be explained with three rays. 1) The ray parallel to the principal axis passes through the focal point after refraction by the lens. 2) The ray passing through the optical center passes straight through the lens and passes undeviated. 3) The ray passing through the focal point becomes parallel to the principal axis after refraction by the lens.

Q 38. When an object is beyond 2F forms a converging lens, which type of image is formed?

Ans: When an object is beyond 2F from the converging lens, the image is between F and 2F, real, inverted, and smaller than the object.

Q 39. When an object is present at 2F which type of image is formed in the case of the convex lens?

Ans: The image is at 2F real inverted and of the same size as the object in the case of a convex lens.

Q 40. When an object is present between lens and f then what is the nature of the image formed by a convex lens?

Ans: When an object is between lens and F the image is formed behind the object, virtual, erect, and larger than the object in the case of a convex lens.

Q 41. What is meant by lens formula?

Ans: The relation between the object and image distance from the lens in terms of the focal length of the lens is called the lens.

Q 42. What do you know about periscope?

Ans: A Periscope is a long tube at the two ends of which are adjusted two reflecting prisms. The rays of light from an object are reflected through an angle of 900 by each prism. Thus light from a viewed object enters the observer’s eye. Periscope is normally used in tanks and submarines. With the help of a periscope, the commander of the submarine sitting in his cabin can see the surface of the sea. Similarly, a soldier sitting inside a tank can see the objects outside the tank.

Q 43. How the power is lost in optical fiber through dispersion? Explain.

Ans: When a light signal travels along fibers by multiple refractions, some of the light is absorbed by the impurities in the glass. Some of it is scattered by the group of atoms formed at places such as joints when fibers are joined together. Note: Careful manufacturing can reduce power loss by scaling and absorption.

Q 44. What do you understand from Linear and Angular Magnification?

Ans: Linear Magnification: “The ratio of the size of the image to the size of the object is called linear magnification.” Angular Magnification: “The ratio of the angle subtended by the image as seen through the device to that subtended by the object at the unaided eye is known as angular magnification”.

Q 45. What do you know about a totally reflecting prism? Also, write its uses.

Ans: A transparent body, whose three sides are rectangular and two sides are right-angled triangular is called a reflecting prism. Reflecting prism: A prism that has one angle of 90° is called a reflecting prism. Principal of totally reflecting prism: The total reflecting prism works on the principle of total internal reflection. The reflecting prism reflects a beam of light through 90° or 180°. One angle of the right angle and the other two 45° each. When light falls perpendicular to one side it enters the prism without deviation. These light rays strike with the hypogynous of the prism. The hypotenuse at the angle of 45°. This angle is greater than the critical angle of glass which is 42°. So the angle of incidence is greater than the critical angle the lights reflect through a prism at an angle of 90°. Uses of totally reflecting prism: • The reflecting prism is used in periscope. • It is used in binoculars. • it is used in textile engineering for designing. • It is used in projectors.

Q 46. How the light signal is transmitted through optical fiber?

Ans: The light signal is transmitted through the optical fiber in the form of 1 and 0. The digit 1 represents the presence of light while the digit 0 represents the absence of light. Note: usually the light signal is produced by “Laser” or “LED” which travels through the optical fiber.

Q 47. What is a simple microscope?

Ans: A magnifying glass is a convex lens that is used to produce magnified images of small objects. Hence it is called a simple microscope.

Q 48. What is meant by resolving the power of the instrument?

Ans: The resolving power of an instrument is the ability to distinguish between two closely placed objects of point sources.

Q 49. What is a magnifying glass?

Ans: Magnifying glass is a lens that forms a virtual image that is larger than the object and appears behind the lens.

Q 50. What is a compound microscope? Also, write its three features.

Ans: A compound microscope has two converging sets of lenses, the objective, and the eyepiece, and is used to investigate the structure of small objects. Three main Features are as • It gives greater magnification than a single lens. • The objective lens has a short focal length, f < 1 cm. • The eyepiece has a focal length, fe Of a few cm.

Q 51. Which formula is used to determine the magnification of a compound microscope?

Ans: The magnification of the compound microscope.

Q 52. Write uses of a compound microscope.

Ans: A compound microscope is used to study bacteria and other micro-objects. It is also used for research in several fields of science like Microbiology, Botany, Geology, and Genetics.

Q 53. Why would it be advantageous to us when blue light is used with a compound microscope?

Ans: The blue light of short wavelength produces less diffraction increases its resolving power. Hence, it allows more details to be seen.

Q 54. What is the least distance of distinct vision? How it is affected by the increase in age?

Ans: The minimum distance of an object from the eye at which it produces a sharp image on the eye is called the least distance of a distinct or near point. Its volume is normally 25 cm with the increase in age, its value usually increases.

Q 55. Why objective of a short focal length is preferred in a microscope?

Ans: The magnification of the microscope. It is a chair from the above equation that M α 1 for i.e. smaller to focal length of objective greater will be its magnifying power and vice versa. Therefore, to increase the magnification power the value of the focal length of the objective must be smaller.

Q 56. Define telescope. What do you know about refracting telescopes?

Ans: Telescope is an optical instrument that is used to observe distant objects using lenses or mirrors. A telescope that uses two converging lenses is called a refracting telescope.

Q 57. How terrestrial telescope is different from a telescope?

Ans: The terrestrial telescope has an extra lens between the objective and the eyepiece.

Q 58. What is a magnification of a refracting telescope?

Ans: Magnification of refracting telescope can be determined by using the formula:

Q 59. Which human organ works like the camera?

Ans: The human eye works like a camera.

Q 60. What is a pupil?

Ans: Iris has an opening at its center called the pupil.

Q 61. What is meant by a defect of vision? What are its types?

Ans: The inability of the eye to see the image of objects is called a defect of vision. The defects of vision are: • Nearsightedness (Myopia) • Farsightedness(Hypermetropia).

Q 62. What is the reason for short-sightedness? How It is corrected?

Ans: Shortsightedness is due to the eyeball being too long. Light rays from a distant object are focused in front of the retina and a blurred image is produced. It is corrected by using, a diverging lens.

Q 63. What is meant by hyper-metropia? How is it corrected?

Ans: The disability of the eye to form distinct images of nearby objects on its retina is known as farsightedness or hypermetropia. This defect can be corrected with the aid of a suitable converging lens.

Q 64. What are the near and far points of the normal eye?

Ans: Near Point: A normal eye can see near objects clearly at a distance of about 25cm from the eye. This is near the point of the normal eye. Far Point: A normal eye can see far-off objects. So we can say that the far point of a normal eye will be at an infinite distance.

Q 65. Which lens is used for the long-sightedness?

Ans: Long-sightedness is corrected by wearing spectacles having a convex lens of such focal length which forms a virtual image of the object placed at normal 25 cm, at the near point O of the eye. Hence a clear image of the object is formed on the retina.

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