REFLECTION AND PLANE SURFACES

REFLECTION AND PLANE SURFACES

Laws

• The angle of incidence is equal to the angle of reflection
• The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane
• Experiment to verify the law of reflection
• A plane sheet of paper is got
• A horizontal line MM is drawn
• A perpendicular to the line MM/the normal is drawn
• A mirror is placed on the line MM
• A suitable angle I is measured on the line and a line is drawn
• Pins P₁ and P₂ are placed along that line
• Looking into the mirror, pins P₃ and P₄ are placed in line with the images of pins P₁ and P₂
• The pins are removed and a line is drawn through P₃ and P₄
• The angle r is measured and it is found that angle I is equal to angle r
• Diagram
• Types of reflection

There are two types of reflection on surfaces

Regular reflection

Is the type of reflection that occurs on smooth surfaces where the parallel incident rays are reflected as parallel rays.

Diffuse reflection

Is the reflection on rough shiny surfaces where the parallel beam is reflected as a non –parallel beam.

Diagram

Characteristics of images formed in a plane mirror

• They are virtual
• They are laterally inverted
• They are the same size as object
• They are the same distance from mirror as object

If a person is hm tall, the mirror needed to view full size is ½ h

Diagram

Rotation of the reflected ray

Diagram

MIRROR PERISCOPE

It consists of two plan mirrors arranged with reflecting surfaces facing each other and parallel. The plane mirrors are incline at 45⁰to the horizontal. Parallel rays from objects are turned through 90⁰ in each mirror enabling an observer to overcome obstacles and see the object.

Prisms are sometimes used instead of plane mirrors since they produce clear images. The prism totally reflects all energy. They are used in submarines, by mountain climbers.

Diagram

THE SEXTANT

It consists of two mirrors M₁ fully silvered and M₂ is silvered on one side. It is used to measure angle of elevation of a star in order to determine latitude.

PROCEDURE

The horizon H is observed through the unsilvered part of M₂

The image H¹ of the same horizon is observed as a result of reflection in the two mirrors.

The mirror M₁ is turned until the two images coincide, at this point the mirrors are parallel

The position of M₁ is noted on the angle scale

M₁ is turned to observe the image of the star at the horizon H

The position (Theta)₂ of the mirror M₁ is noted and then the angle of rotation (Theta) of the mirror

Angle of elevation of the star (Theta)

Diagram