The camera lens has an aperture off and the exposure time is ( 1/60) s. What will be the new exposure time if the aperture become 1.4 f ?

• $\frac{1}{42}$

• $\frac{1}{56}$

• $\frac{1}{72}$

• $\frac{1}{31}$

A point source is kept at a distance of   1000 m has an illumination I. To change the illumination to 161 the new distance should become

• 250 m

• 500 m

• 750 m

• 800 m

Assertion:  Goggles have zero power.

Reason: Radius of curvature of both sides of lens is same.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

Assertion:  Optical fibres are used for telecommunication.

Reason: Optical fibres are based on the phenomenon of total internal reflection.

• If both assertion and reason are true and reason is the correct explanation of assertion

• If both assertion and reason are true but reason is not the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

115.

In refraction, light waves are bent on passing from one medium to the second medium, because in the second medium

• the frequency is different

• the coefficient of elasticity is different

• the speed is different

• the amplitude is smaller

A wire mesh consisting of very small squares is viewed at a distance of 8 cm through a magnifying converging lens of focal length 1O cm, kept close to the eye. The magnification produced by the lens is

• 5

• 8

• 10

• 20

A lens is made of flint glass (refractive index= 1.5). When the lens is immersed in a liquid of refractive index 1.25, the focal length

• increases by a factor of 1.25

• increases by a factor of 2.5

• increases by a factor of 1.2

• decreases by a factor of 1.2

Assertion:  In optical fibre, the diameter of the core is kept small. 

Reason:  This smaller diameter of the core ensures that the fibre should have incident angle more than the critical angle required for total internal reflection.

• If both assertion and reason are true and reason is the correct explanation of the assertion

• If both assertion and reason are true but reason is not the correct explanation of the assertion

• If assertion is true, but reason is false

• Both assertion and reason are false statements

What should be the maximum acceptance angle at the air-core interface of an optical fibre if  n₁  and  n₂ are the refractive indices of the core and the cladding, respectively?

• sin^-1( n₂/n₁ )

• sin^-1$\sqrt{{\mathrm{n}}_1^{ 2} - {\mathrm{n}}_2^{ 2}}$

• $\left[{\tan }^{-1} \frac{{\mathrm{n}}_2}{{\mathrm{n}}_1}\right]$

• $\left[{\tan }^{-1} \frac{{\mathrm{n}}_1}{{\mathrm{n}}_2}\right]$

A telescope has an objective lens of focal length 200 cm and an eye piece with focal length 2 cm. If this telescope is used to see a 50 meter tall building at a distance of 2 km, what is the height of the image of the building formed by the objective lens?

• 5 cm

• 10 cm

• 1 cm

• 2 cm