Question 34: Ray Optics

Question

In parallax method for the determination of focal length of a concave mirror, the object should always be placed:

Accepted Answer

The parallax method is an experimental technique used to locate the exact position of an image. It involves using an object pin and an image pin. The position of the image is found by placing the image pin such that there is no relative shift (no parallax) between the image pin and the actual image of the object pin when you move your eye side to side.
This technique requires the image to be a real image that is formed in the actual space in front of the mirror. So, to use the parallax method, the concave mirror is required because it produces a real image.
The image formation cases for a concave mirror based on the object's position :
Between the Pole (P) and Focus (F) :
The mirror forms a virtual, erect, and magnified image behind the mirror. A virtual image cannot be located using a physical image pin in standard parallax methods because the light rays do not actually converge at the image location.
At the Focus (F) : The image is formed at infinity.
Between F and C : The image so formed is real and inverted in front of mirror.
At C : The mirror forms a real and inverted image in front of the mirror.
While placing the object between F and C does form a real image, stating it ONLY works here is incorrect because placing it beyond C also works. Hence, option (A) is incorrect.
Placing the object between P and F forms a virtual image, making the standard parallax method impossible. Hence, option (B) is incorrect.
Placing the object at any point beyond the focus (F) the nature of image formed is real (whether it is formed beyond C , at C , or between C and F ). This allows the image pin to be used to remove parallax. Hence,
While placing it beyond C forms a real image, restricting it to ONLY beyond C is incorrect because placing it between F and C also works. Hence, the option (D) is incorrect.
Therefore, the correct option is (C).

Answer

between the focus $(F)$ and the centre of curvature $(C)$ of the mirror ONLY

Answer

between the pole $(P)$ and the focus $(F)$ of the concave mirror ONLY

Answer

at any point beyond the focus $(F)$ of the mirror

Answer

beyond the centre of the curvature $(C)$ of the mirror ONLY

Ray Optics

In parallax method for the determination of focal length of a concave mirror, the object should always be placed:

Choose the correct answer: