Question

An object at a distance 30cm from a concave mirror gets its image at the same point. What is the focal length of mirror

Answer 1

The focal length of mirror is 15 cm.

Explanation:

To calculate the focal length use the mirror formula as

$\frac{1}{f}=\frac{1}{u} +\frac{1}{v}$

Here u is the object distance and v is the image distance.

As object and image are located at the same point and in front of concave mirror then we take u = +30 cm and v = + 30 cm.

Substitute the values, we get

$\frac{1}{f} =\frac{1}{30cm} +\frac{1}{30cm}$

$\frac{1}{f} =\frac{2}{30cm}$

f = + 15 cm.

Thus, the focal length of mirror is 15 cm.

#Learn More: Mirror formula

Answer 2

Answer:

Object distance(u)=-30cm

Image distance(v)=-30cm

Focel length(f)=?

From mirror equation

1/f=1/u + 1/v

1/f=(- 1/30)+(-1/30)

1/f=-1/15

f=-15cm

Therefore, focel length=-15cm

So, its a concave mirror where obect is placed at centre of curvature.

Explanation:

For any spherical mirror the object distance is always negetive since the object is placed on the left from the pole.