CBSE Class 12 Physics HOTs Optics Set A

ONE MARK QUESTIONS

Question. An object is held at the principal focus of a concave lens of focal length f. Where is the image formed? 
Answer : The image will be formed between optical centre and focus of lens; towards the side of the object.

Question. A diverging lens of focal length ‘F’ is cut into two identical parts each forming a plano-concave lens. What is the focal length of each part? 
Answer : Focal length of each half part will be twice the focal length of initial diverging lens.

Question. When light travels from a rarer to a denser medium, the speed decreases. Does this decrease in speed imply a decrease in the energy carried by the light wave? Justify your answer.
Answer : No; when light travels from a rarer to denser medium, its frequency remains unchanged.
According to quantum theory, the energy of a light beam depends on frequency and not on speed.

Question. Two thin lenses of power +6 D and –2 D are in contact. What is the focal length of the combination? 
Answer : Net power of lens combination P = P1+ P2= + 6 D - 2 D = + 4 D
f( in m) = ¼ ; f = 25 cm

Question. A converging lens is kept co-axially in contact with a diverging lens – both the lenses being of equal focal lengths. What is the focal length of the combination?
Answer : Power of combination P = P1 + P2 = 1/f - 1/f = 0
∴ Focal length of combination f = 1/P = 1/0 = ∞

TWO MARKS QUESTIONS

Question. (i) State the principle on which the working of an optical fiber is based.
(ii) What are the necessary conditions for this phenomenon to occur?
Answer : (i) The working of optical fiber is based on total internal reflection
Statement: When a light ray goes from denser to rarer medium at an angle greater than critical angle, the ray is totally reflected in first (denser) medium. This phenomenon is called total internal reflection.
(ii) Conditions :
(a)Angle of incidence must be greater than critical angle (i. e., i > C).
(b) ray must go from denser to rarer medium

Question. (i) What is the relation between critical angle and refractive index of a material?
(ii) Does critical angle depend on the colour of light? Explain.
Answer : (i) Relation between refractive index (n) and critical angle (C) is n= 1 / sin C
(ii) Yes, critical angle depends on wavelength or colour of light; it increases with increase of wavelength being maximum for red and minimum for violet.

Question. An object AB is kept in front of a concave mirror as shown in the figure.
Complete the ray diagram showing the image formation of the object.= tan^-1⁡μ= tan^-1⁡μ
How will the position and intensity of the image be affected if the lower half of the mirror’s reflecting surface is painted black?
Answer : (i) Image formed will be inverted diminished between C and F.

(ii) No change in position of image and its intensity

Question. Draw a labelled ray diagram of a reflecting telescope. Mention its two advantages over the refracting telescope.
Answer : Newtonian Telescope: (Reflecting Type) 

Advantages:
(i) It is free from chromatic and spherical aberrations.
(ii)Its resolving power is greater than refracting telescope due to larger aperture of mirror.

Question. Draw a schematic ray diagram of a compound microscope when image is formed at distance of distinct Vision .Write the expression for resolving power of a compound microscope. How can the resolving power of a microscope be increased ? 
Answer : 

R.P. =2n Sinθ/1.22λ
R.P. can be increased by increasing the numerical aperture or by decreasing the wave length.

Question. A ray PQ incident normally on the refracting face BA is refracted in the prism BAC made of material of refractive index 1.5. Complete the path ray through the prism. From which face will the ray emerge ? justify your answer.
Answer : 

FIVE MARKS QUESTIONS

Question. Draw a schematic ray diagram of a compound microscope. Write briefly its working. Derive magnifying power.
Why must both the objective and eye piece of a compound microscope have short focal length?
Answer : (Image 10) When both the objective and eye piece of a compound microscope have small focal length, magnifying power will be large.

Question. (a)Draw a labelled ray diagram to obtain the real image formed by an astronomical telescope in normal adjustment position its magnifying power. (b) You are given there lenses of power 0.5 D, 4D and 10D to design a telescope.(i)Which lenses should he used as objective and eyepiece ? Justify your answer.
(ii) Why is the aperture of the objective preferred to be large ?
(iii)Light from a point source in air falls on a convex spherical glass surface of refractive index
1.5 and radius of curvature 20 cm. The distance of light source from the glass surface is 100
cm. At what position is the image formed ?
Answer :  

(b) (i) 0.5 D for Objective lens and 10 D for eyepiece because M = ffe0
(ii) To collect sufficient light to from a bright image 

Question. Draw a schematic ray diagram of a compound microscope when image is formed at distance of distinct Vision .Write the expression for resolving power of a compound microscope. How can the resolving power of a microscope be increased ?
Answer : Diagram 

R.P. =2n Sinθ/1.22λ
R.P. can be increased by increasing the numerical aperture or by decreasing the wave length.