Question 229: Ray Optics

Question

A convex lens is put 10 cm from a light source and it makes a sharp image on a screen, kept 10 cm from the lens. Now a glass block (refractive index 1.5) of 1.5 cm thickness is placed in contact with the  light source. To get the sharp image again, the screen is shifted by a distance d. Then d is :

Accepted Answer

Image is formed on the screen.
So, v = 10 cm
and  $$\mu $$ =  $$-$$ 10 cm
Using formula,
$${1 \over v} - {1 \over u} = {1 \over f}$$
$$ \Rightarrow $$   $${1 \over {10}} - {1 \over {\left( { - 10} ight)}}$$ = $${1 \over f}$$
$$ \Rightarrow $$   f = 5 cm
Now a glass block  is placed  like this,
Because of this glass block source will move t$$\left( {1 - {1 \over \mu }} ight)$$ in the direction of  incident ray.
$$ 	herefore $$   S' = t$$\left( {1 - {1 \over \mu }} ight)$$
= 1.5$$\left( {1 - {2 \over 3}} ight)$$
=  0.5
$$ 	herefore $$   now distance of source from the lens = 10 $$-$$ 0.5  =  9.5 cm
$$ 	herefore $$   $$\mu $$  =  $$-$$ 9.5 cm
$$ 	herefore $$   $${1 \over v} - {1 \over {\left( { - 9.5} ight)}}$$  =  $${1 \over 5}$$
$$ \Rightarrow $$   $${1 \over v}$$  =  $${1 \over 5} - {2 \over {19}}$$
$$ \Rightarrow $$   $${1 \over v}$$ =  $${9 \over {95}}$$
$$ \Rightarrow $$   v  =  10.55 cm
So, to get sharp image screen should shift away (10.55 $$-$$ 10) = 0.55 cm from the lens.

Answer

1.1 cm away from the lens

Answer

0

Answer

0.55 cm towards the lens

Answer

0.55 cm away from the lens

Ray Optics

A convex lens is put 10 cm from a light source and it makes a sharp image on a screen, kept 10 cm from the lens. Now a glass block (refractive index 1.5) of 1.5 cm thickness is placed in contact with the light source. To get the sharp image again, the screen is shifted by a distance d. Then d is :

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