Reflection of Light Calculator

Apply the law of reflection to calculate reflection angle, ray separation, and mirror rotation effects.

θr = θi

Angle of incidence θi (degrees)

Mirror rotation (degrees)

Enter valid values in all required fields.

What this formula means

The law of reflection states that the angle of reflection equals the angle of incidence when both are measured from the surface normal. Incident ray, reflected ray, and normal lie in the same plane. Rotating a plane mirror by a small angle rotates the reflected ray by twice that angle.

Use this relationship in mirrors, periscopes, laser alignment, optical scanners, surveying instruments, and ray diagrams. The result should be interpreted with consistent units and with the assumptions of the underlying model in mind.

Worked example

For an incidence angle of 35°, the reflection angle is 35° and the angle between the two rays is 70°.

Applications

Use this relationship in mirrors, periscopes, laser alignment, optical scanners, surveying instruments, and ray diagrams. It is also useful as an independent check when comparing hand calculations, laboratory measurements, simulations, or preliminary engineering estimates.

Assumptions and common mistakes

Important: Measure angles from the normal, not from the mirror surface. An angle measured from the surface is the complement of the incidence angle.

Use SI units unless the field label states otherwise. Keep significant figures appropriate to the accuracy of the inputs, and do not treat a simplified calculator as a replacement for codes, detailed design standards, or professional review.

Frequently asked questions

What equation does this calculator use?θr = θi. The calculator evaluates the displayed relationship using the values entered above.

Are the results unit-sensitive?Yes. The input labels use SI units so the displayed output follows the corresponding SI unit.

Can I use this for engineering design?Use it for learning and preliminary checks. Final designs require validated data, applicable standards, safety factors, and professional judgment.