Which of the following figures will depict deviation of a ray of light through 90° when it emerges out of the prism.

1.

2.

3.

4.

option (1)

Reason — When light passes through a right-angled isosceles prism, and is incident perpendicularly on one of its sides, it can undergo total internal reflection (TIR) at the hypotenuse and finally emerge at 90° from the original direction.

Such a situation occurs when :

• The light enters one perpendicular face,
• Reflects off the hypotenuse (TIR at 45°), and
• emerges from the other perpendicular face, making a 90° turn overall.

So, a prism satisfying above conditions will definitely produce a ray deviated by 90°.

Now,

In option (1)

• Isosceles Right-angled triangle.
• Light enters perpendicularly to one of the perpendicular sides.
• Will reflect off the hypotenuse and exit from the other perpendicular side as shown in below figure.

Hence, it deviates the incident ray by 90°.

In option (2)

• Isosceles triangle with the vertex angle at the top being 90°.
• Light enters symmetrically from one side.
• Due to symmetry, this setup generally leads to no net deviation as shown in below figure.

Hence, it can not deviate the incident ray by 90°.

In option (3)

• Isosceles Right-angled triangle.
• Ray enters at the hypotenuse, perpendicularly.
• Emerges from the hypotenuse again and deviates the ray by 180° as shown in below figure.

Hence, it can not deviate the incident ray by 90°.

In option (4)

• Triangle with 60°, 90°, and 30° angles.
• Light enters at the face opposite 60°.
• Not a right-angled isosceles prism.
• Path will not lead to 90° deviation as shown in below figure.

Hence, it can not deviate the incident ray by 90°.