In a concave mirror, an object is placed at a distance  d₁  from  the focus  and  the image is formed  at  a distance d₂ from the focus. Then the focal length of the mirror is

• $\sqrt{{\mathrm{d}}_1 {\mathrm{d}}_2}$

• d₁ d₂ 

• (d₁ + d₂ )/ 2

• $\sqrt{\raisebox{1ex}{${\mathrm{d}}_1$}\!\left/ \!\raisebox{-1ex}{${\mathrm{d}}_2$}\right.}$

A short linear object, of length l, lies along  the  axis  of  a  concave  mirror, of focal length f, at a distance d from the pole of the mirror. The  size  of  the  image  is  then (nearly)  

• $- \frac{\mathrm{l} \mathrm{f}}{\mathrm{d} - \mathrm{f}}$

• $\frac{\mathrm{d} - \mathrm{f}}{\mathrm{l} \mathrm{f}}$

• $\mathrm{l} \frac{{\mathrm{f}}^2}{{\left(\mathrm{d} - \mathrm{f}\right)}^2}$

• $\frac{{\left(\mathrm{d} - \mathrm{f}\right)}^2}{{\mathrm{f}}^2} \mathrm{l}$

273.

Assertion:  A  total  reflecting  prism  is  used  to erect  the  inverted  image  without deviation. 

Reason:  Rays  of  light  incident  parallel  to  base  of  prism  emerge  out  as  parallel rays.

• 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: The  edges  of  the  images  of  white  object  formed  by  a  concave  mirror  on  the  screen  appear white. 

Reason:  Concave  mirror  does  not  suffer  chromatic aberration.

• 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

A coverging lens forms a real image I of an object on its principal axis. A rectangular slab of refractive index and thickness  x  is introduced between  l  and the lens, I will move

• towards the lens ( μ - 1 ) x

• towards the lens by $\left(1 - \frac{1}{\mathrm{\mu }}\right)$ x

• away from the lens by (μ - 1 ) x

• away from the lens by $\left(1 - \frac{1}{\mathrm{\mu }}\right)$ x

When white light passes through a prism, the deviation is maximum for

• violet light

• green light

• red light

• yellow light

An object 5 cm tall is placed 1 m from a concave spherical mirror which has a radius of curvature of 20 cm. The size of the image is

• 0.11 cm

• 0.50 cm

• 0.55 cm

• 0.60 cm

The magnifying power of a compound microscope increase with

• the focal length of objective lens is increased and that of eye lens is decreased

• the focal length of eye lens is increased and that of objective lens is decreased

• focal lengths of both objects and eye-piece are increased

• focal lengths of both object and eye-piece

A thread is tied slightly loose to a wire frame as in figure and the frame is dropped into a soap solution and taken out. The frame is completely covered with the film. When the portion A is punctured with a pin, the thread

• become concave towards A

• become convex toward A

• either (a)  or  (b) depending on the size of A with respect to B

• remains in the initial position

Assertion: The colour of the green flower seen through red glass appears to be dark

Reason: Red glass transmits only red light.

• 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