Question

Write the equation down for the thin lens. How is this equation related to the lens maker equations, are any approximations made to reduce the lens maker equation to the thin lens equation? Discuss.

When light enters and exits the lens, the rays are shown in the diagrams in the laboratory write up as bending. What phenomenon accounts for the bending of the light moving from air to glass and then again from glass to air?

A water droplet can be used as a magnifier. Discuss your ideas as to why the water droplet can act like a lens.

Answer 1

Equation for thin lens is

where di is image distance from optical center

do is object distance from optical center

f is focal length of lens

Lens maker equation is

Here f is focal length

n is refractive index of lens with respect to surrounding medium

R1 and R2 are radii of curvature of two sides of lens

To reduce lens maker equation to thin lens equation:

1. Lens is considered to be thin. any optical effect due to thickness of lens is ignored

2. Rays incident on lens are considered paraxial i.e the angle made by rays with principal axis is considered very small.

Light bends when it travels from one medium to another because there is change in speed of light. Due to change in speed of light it takes path of least time. According to Fermat's principle light always take a path of least time.

Water drop has surface which is convex in nature. Due to convex surface it behaves like a convex lens. Water drop when placed over anything forms its magnified image. This is because at that time the object is placed between optical center and focus of lens. When object is located at this position, then water drop forms its virtual, erect and magnified image.