## A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source …

Q: A beam of light from a source L is incident normally on a plane mirror fixed at a certain distance x from the source . The beam is reflected back as a spot on a scale placed just above the source L . When mirror is rotated through a small angle θ , the spot of light is found to move through a distance y on the scale . The angle θ is given by :

(a) y/x

(b) x/2y

(c) x/y

(d) y/2x

Ans: (d)

Sol: When mirror is rotated by angle θ ,the reflected ray is turned by 2θ .

2θ = y/x

θ = y/2x

## An astronomical refracting telescope will have large angular magnification and high angular resolution…

Q: An astronomical refracting telescope will have large angular magnification and high angular resolution , when it has an objective lens of

(a) large focal length and large diameter

(b) small focal length and small diameter

(c) large focal length and small diameter

(d) small focal length and large diameter

Ans: (a)

Sol: Angular magnification $\displaystyle m = \frac{f_o}{f_e}$

Angular resolution $\displaystyle \theta = \frac{D}{1.22 \lambda}$

## An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm . If the object is displaced through…

Q: An object is placed at a distance of 40 cm from a concave mirror of focal length 15 cm . If the object is displaced through a distance of 20 cm towards the mirror , the displacement of the image will be

(a) 30 cm towards the mirror

(b) 36 cm towards the mirror

(c) 36 cm away from the mirror

(d) 30 cm away from the mirror

Ans: (c)

Sol: (1/v1) + (1/-40) = (1/-15)

v1 = -24 cm

(1/v2) + (1/-20) = (1/-15)

v2 = -60 cm

Shift = 60 – 24 = 36 cm , away from mirror

## The image produced by a concave mirror is one- quarter the size of object is moved 5 cm closer to the mirror…..

Q.The image produced by a concave mirror is one- quarter the size of object , if moved 5 cm closer to the mirror, the image will only be half the size of the object. The focal length of mirror is

(a) f = 5.0 cm

(b) f = 2.5 cm

(c) f = 7.5 cm

(d) f = 10 cm

Ans: B

## A ray of light travelling in glass (μ = 3/2) is incident on a horizontal glass-air surface at the critical angle θc …..

Q.A ray of light travelling in glass (μ = 3/2) is incident on a horizontal glass-air surface at the critical angle θc. if a thin layer of water (μ = 4/3) is now poured on the glass-air surface, the angle at which the ray emerges into air the water-air surface is

(a) 60°

(b) 45°

(c) 90°

(d) 180°

Ans: (c)

Sol: For glass – air interface

$\mu_g sin\theta_c = \mu_a sin90$ ..(i)

For glass -water interface

$\mu_g sin\theta_c = \mu_w sin\theta_1$ …(ii)

For water -air interface

$\mu_w sin\theta_1 = \mu_a sin\theta_2$ …(iii)

Solving (i) , (ii) & (iii)

$\theta_2 = 90°$