Question

Where must be the object placed in front of a convex lens so that the image formed is of same size as the object: (Given focal length for lens = $F$)
A. F
B. 2F
C. 0
D. \[\infty \]

Answer 1

Hint: Refraction is the change in the direction of the light rays when it passes from one medium to another. The image formed by the concave and convex lenses are based on the phenomena of refraction of light. Light rays from the object pass through the lens and change its direction which later result in the formation of an image of that object.

Complete answer:
In a convex lens, the image formed at a point where at least two refracted light rays from the lens meet. We have the following rules for obtaining images formed by the convex lens:
Rule 1: A ray of light parallel to the principal axis passes through the focus of the lens after refraction.
Rule 2: A ray of light passing through the optical centre of the convex lens goes on straight away without any deviation in its path.
Rule 3: When a ray of light passes through the focus of the convex lens then it turns out parallel to the principal axis after refraction through the lens.

When the object is placed at a distance equal to twice the length of focal length i.e. $2F$ of the convex lens then one of the rays of light becomes parallel to the principal axis and passes through the focus of the lens on the other side after the refraction. Another ray of light passes through the optical centre and goes straight after refraction. Both the refracted light rays meet at $2F$ on the other side. The image formed in this case is Real and inverted, the same size as that of the object.

Hence option B is correct.

Note: Convex lens is also known as converging lenses. This lens is used majorly in microscopes, small telescopes as well as in astronomical telescopes. Images formed by the convex lens have the similar properties as the image formed by the concave mirror. Questions may vary by asking the position and the properties of the image formed by the convex lens by placing objects at different positions.