Light Refraction Exercise

Light Refraction Exercise

Exercise # 1

Short Answer Type Question.

 Q.1       What is the maximum angle of refraction when a ray of light is refracted from glass into air ?

 Q.2       What should be the position of an object relative to biconvex lens so that this lens behaves like a magnifying glass?

 Q.3       Can the absolute refractive index of a medium be less than unity?

 Q.4       To a fish under water viewing obliquely a fisherman standing on the bank of a lake, does the man look taller or shorter than what actually he is?

 Q.5       Does the apparent depth of a tank of water change if viewed obliquely? If so, does the apparent depth increase or decrease?

 Q.6       A substance has critical angle of 45° for yellow light. What is its refractive index?

 Q.7       What is critical angle for a material of refractive index ?

 Q.8       A ray of light is incident normally on a glass slab. What is the angle of refraction?

 Q.9       What is the power of the combination of a convex lens and a concave lens of the same focal length?

 Q.10     How is power of a lens related to its focal length?

Q.11     Define critical angle for total internal reflection.

Short Answer Type Question.

Q.12     Explain the shining of an air bubble in water.

 Q.13     For the same angle of incidence, the angles of refraction in media P, Q and R are 35°, 25°,15° respectively, In which medium will the velocity of light be minimum ?

 Q.14     Define focus and principal focus of a lens.

 Q.15     A virtual image, we always say, cannot be caught on a screen. Yet when we ‘see’ a virtual image, we are obviously bringing it on to the ‘screen’ i.e., retina of our eye. Is there a contradiction?

 Q.16     A convex lens is held in water. What would be the change in the focal length ?

 Q.17     Why goggles (Sun glasses) have zero power even though their surfaces are curved?

Q.18     The lens shown in fig. is made of two different materials. A point objects is placed on the principal axis of this lens. How many images will be obtained?

Q.19     Refer to fig. (a), (b) and (c). Give relationship between m1 and m2 in each case.

Q.20     Images formed by totally reflected light are brighter than the images formed by ordinary reflected light. why ?

Q.21     Can light travelling from air to glass suffer total internal reflection? Justify your answer.

 

Q.22     What are the five general features of the image formed by a plane mirror ?

 

 

Q.23     (a) What is total internal reflection? How is critical angle related to refractive index?

(b) A ray of light while travelling from a denser to a rarer medium undergoes total internal reflection. Derive the expression for the critical angle in terms of the speed of light in the respective media.

 

Q.24     Discuss in detail refraction at

(i) convex surface     (ii) concave surface.

 

Q.25     Derive lens formula for a thin lens.

 

Q.26     What is critical angle? Give one application of total internal reflection.

 

Q.27     State Snell’s law of refraction.

 

Q.28     What is meant by power of a lens? What is

one dioptre ?

 

Q.29     The speed of light in air is 3 × 108 m/s. Calculate the speed of light  in glass given that the refractive index of glass is 1.5.

 

Q.30     The refractive index of water with respect to air is 4/3. Calculate the refractive index of air with  respect to water.

 

Q.31     The refractive index of glass is 3/2. What is the critical angle for the glass-water surface?

 

Q.32     A ray of light travelling in air falls on the surface of a glass slab at an angle of incidence 45°. Find the angle made by the refracted ray with the normal within the slab where refractive index for glass is 3/2.

Q.33     A ray of light travelling in air is incident on the surface of a transparent material of refractive index . If the angle of refraction is 30°, calculate the angle of incidence.

Q.34     Focal length of a convex lens is 50 cm. Calculate its power.

Q.35     A point object is placed at a distance of 12 cm from a convex lens on its principal axis. Its image is formed 18 cm from the lens on the other side. Calculate the focal length of the lens.

 

Q.36     An object is placed at a distance of 20 cm from a concave lens on its principal axis. If the focal length of the lens is 20 cm, find the position of the image.

 

Q.37     A beam of light incident parallel to the principal axis of a concave lens appears to diverge from a point 20 cm behind the lens after refraction through the lens. Calculate the power of the lens.

 

Q.38     A pin 2 cm long is placed at a distance of 16 cm from a convex lens of focal length 12 cm perpendicular to the principal axis. Find the position, nature and size of the image.

 

Q.39     A convex lens of focal length 20 cm and a concave lens of focal length 12.5 cm are placed in contact having the same principal axis. Calculate the power of the combined lens.

 

Q.40     Two thin lenses of power +3.5D and –2.5D are placed in contact. Find the power and focal length of the lens combination.

 

Q.41     An illuminated slit is kept at a distance of 40 cm in front of a convex lens of focal length 15 cm. Find the position of the screen to obtain the image.

 

Q.42     A ray incident at a slab at angle 10° as shown in figure. Find angle of emergent q.

 

 

 

 

Exercise # 2

 

 

 

Q.1       How will the image formed by a convex lens be affected, if the central portion of the lens is wrapped in black paper, as shown in the fig.

 

(A) No image will be formed

(B) Full image will be formed but it is less bright

(C) Full image will be formed but without the central portion

(D) Two images will be formed, one due to each exposed half.

 

 

Q.2       The critical angle for light going from medium X into medium Y is q. The speed of light in medium X is u. The speed of light in medium Y is-

(A) u (1 – cos q)        (B) u/cos q

(C) u cos q                (D) u/sin q

 

Q.3       One surface of a lens is convex and the other is concave. If the radii of curvature are r1 and r2 respectively, the lens will be convex, if-

(A) r1 > r2                 (B) r1 = r2

(C) r1 < r2                 (D) r1 = 1/r2

 

Q.4       An object is immersed in a fluid. In order that the object becomes invisible, it should

(A) behave as a perfect reflector

(B) absorb all light falling on it

(C) have refractive index one

(D) have refractive index exactly matching with that of the surrounding fluid.

 

Q.5       R.I. of glass w.r.t. air is , then the R.I. of air w.r.t. glass is-

(A)        (B)       (C)         (D) 3

 

Q.6       Refractive index of glass with respect to air is 1.5 and refractive index of water with respect to air is . What will be the refractive index of glass with respect to water ?

(A) 1                        (B) 1.5

(C) 1.125                  (D) –10

 

Q.7       The refractive index of a medium depends upon-

(A) Nature of material of the medium

(B) Optical density of the medium

(C) Wavelength of light

(D) All of these

 

Q.8       If refractive index of water w.r.t. air is , then refractive index of air w.r.t. water will be-

(A) 4 × 3    (B)       (C)      (D)

 

Q.9       A ray of light is incident normally on a rectangular piece of glass. The value of angle of refraction will be-

(A) 180°    (B) 90°     (C) 45°      (D) 0°

 

Q.10     What is the angle of deviation ?

(A) Angle between the reflected ray and incident ray

(B) Angle between the reflected ray and refracted ray

(C) Angle between the incident ray and refracted ray

(D) angle between the incident ray and emergent ray

 

Q.11     The speed of light in vacuum is 3.0 × 108 m/s. If the refractive index of a transparent liquid is 4/3, then the speed of light in the liquid is-

(A) 2.25 × 108 m/s      (B) 3 × 108 m/s

(C) 4 × 108 m/s          (D) 4.33 × 108 m/s

 

Q.12     A swimming pool appears to be 2m deep. Its actual depth is (m for water  = 1.33)-

(A) 2.66 m                (B) 2 m

(C) 2.34                    (D) 2.54 m

 

Q.13     To get a real and inverted image of the same size as that of the object the object should be placed in front of the convex lens at-

(A) F

(B) 2F

(C) between F and 2F

(D) away from 2F, where F is focus

 

Q.14     A spherical mirror and a spherical lens each have focal length of –10 cm. The mirror and lens are-

(A) both convex

(B) both concave

(C) mirror is convex and lens is concave

(D) mirror is concave and lens is convex

 

Q.15     The power of a lens having focal length 50 cm is-

(A)     (B) 2D      (C) 3D       (D) 0.2 D

 

Q.16     The focal length of a lens of power –2.0 D is-

(A) –2.0 m                (B) 0.2 m

(C) –0.5 m                 (D) 0.5 m

 

Q.17     Two lenses of power + 5D and –5D are placed in close contact. The focal length of the combination is-

(A) Zero                    (B) ¥

(C) Zero or ¥             (D) None of these

 

Q.18     A student needs a lens of power –2.0 diopter to correct his distant vision. The focal length of the given lens is-

(A) +50 cm                (B) –50 cm

(C) 100 cm                (D) –100 cm

 

Q.19     Focal length of coloured goggles (Without number) is-

(A) zero

(B) infinity

(C) between zero and infinity

(D) None of these

 

Q.20     Where should an object be placed so that a real and inverted image of very large size is obtained, using a convex lens ?

(A) At the focus         (B) At 2F

(C) Between F and 2F  (D) Beyond 2F

Q.21     A convex lens is –

(A) Thicker at the middle, thinner at the edges

(B) Diverging

(C) Thicker at the edges thinner in the middle

(D) Of uniform thickness everywhere

Q.22     A glass rod of refractive index 1.42 is immersed in kerosene. The refractive index of kerosene is 1.42. Then the rod will-

(A) appear bent

(B) appear raised above the liquid

(C) become invisible

(D) none of the above

 

Q.23     The power of a lens whose focal length is 25 cm is-

(A) 4 Diopter             (B) 25 Diopter

(C) 0.04 Diopter         (D) 2.5 Diopter

 

Q.24     A thin lens is made with a material having refractive index m = 1.5. Both the sides are convex. It is dipped in water (m = 1.33). It will behave like-

(A) convergent lens    (B) a divergent lens

(C) a rectangular slab  (D) a prism

 

Q.25     Choose the correct option-

(A) If the final rays are converging, we have a real image

(B) If the incident rays are converging, we have a real image

(C) If the image is virtual, the corresponding object is called a virtual object

(D) The image of a virtual object is called a virtual image

 

Q.26     A convex lens forms a real image of a point object placed on its principal axis. If the upper half of the lens is painted black.

(A) the image will be shifted backward

(B) the image will not be shifted

(C) the intensity of the image will decrease

(D) both (B) and (C)

 

Q.27     The minimum distance between an object and its real image formed by a convex lens of focal length f is-

(A) f          (B) 2f       (C) 3f        (D) 4f

 

 

 

           

 

 

ANSWER KEY

 

Exercise-1

 

  1. 2 × 108m/s             30. 3/4                     31. 42º                    32. 28º              33. 60º              34. + 2D
  2. 7.2 cm 36. 10 cm on the same side of lens            37. –5D
  3. At 48 cm from the lens on the other side. Image is real, inverted and of size 6 cm.
  4. –8D 40. + 1D, 100 cm      41. 24 cm                42. q = 10°

 

 

Exercise-2