Question

an object of 5cm in size is placed at a distance of 20 cm from it concave mirror of focal lenght 15 cm at what distance from the mirror should a screen be placed to get the sharp image ? also calculate the size of the image​

Answer 1

Given:-

• The height of object (h¹) = 5cm
• Distance from the object (u)= -20cm
• Concave mirror of focal length (f) = -15cm

To Find :-

• The size or height of image (h²) = ?

Solution:-

By applying formula we can calculate the size of image as given in the question that. An object of 5cm in size is placed at a distance of 20 cm from it concave mirror of focal length of 15cm .

Calculation begin:-

• Formula Used:-

⟹ 1/v + 1/u = 1/f

⟹ 1/v + 1/(-20) = 1/(-15)

⟹ 1/v = 1/20 – 1/15

⟹ 1/v = -5/300

⟹ V = -60 cm

The screen should be placed 60 cm in front of the mirror.

• Now calculate the size of image :-

⟹ h¹/h² = - (v/u)

⟹ h²/5 = - (-60)/(-20)

⟹ h² = - 15 cm

Hence,

• The height of image ( h²) = 15 cm
• The screen placed at the mirror (v) = 60cm

Answer 2

Given :-

An object of 5cm in size is placed at a distance of 20 cm from it concave mirror of focal lenght 15 cm

To Find :-

Distance from the mirror should a screen be placed to get the sharp image ? also calculate the size of the image​

Solution :-

1/u + 1/v = 1/f

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

-1/20 + 1/v = -1/15

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

1/v = 1/20 - 1/15

1/v = 3 - 4/60

1/v = -1/60

v = -60

Now

m = -v/u = hi/ho

-(-60)/-20 = hi/5

60/-20 = hi/5

60 × 5 = -20hi

300 = -20hi

-15 = hi

[tex][/tex]