REFLECTION OF LIGHT

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REFLECTION OF LIGHT

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Light Travels in Straight Lines

• A ray is the direction or path along which light energy flows. In a diagram, rays are represented by lines with arrowheads.

• A collection of rays is called a beam.

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THE LAWS OF REFLECTION

FIRST LAWS

The incident ray, the reflected ray and the nornal all lie in the same plane

SECOND LAWS

The angle of incidence, I is equal to the angle of reflection, r

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THE LAWS OF REFLECTIONWhen a ray of light strikes a plane mirror, the light ray reflects off the mirror. Reflection involves a change in direction of the light ray. The convention used to express the direction of a light ray is to indicate the angle which the light ray makes with a normal drawn to the surface of the mirror. The angle of incidence is the angle between this normal and the incident ray; the angle of reflection is the angle between this normal and the reflected ray. According to the law of reflection, the angle of incidence equals the angle of reflection. These concepts are illustrated in the animation at the right.

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Image Formation for Plane MirrorsIn the animation above, an object is positioned in front of a plane mirror. The plane mirror will produce an image of the object on the opposite side of the mirror. The distance from the onject to the mirror equal the distance from the image to the mirror. Any person viewing this image must sight at this image position.

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Image Formation in Plane Mirrors

1. Draw the image of the object.

2. Pick one extreme on the image of the object and draw the reflected ray which will travel to the eye as it sights at this point

3. Draw the incident ray for light traveling from the corresponding extreme on the object to the mirror.

4. Repeat steps 2 and 3 for all other extremities on the object.

Distance of the object

Distance of the image

object image

Plane mirror

normalEye

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Check Your Understanding

• Explain why emergency vehicles such as ambulances are often marked on the front hood with reversed lettering (e.g., ECNALUBMA).Answer: AMBULANCE

• If Suzie stands 3 feet in front of a plane mirror, how far from the person will her image be located?

Answer: 6 feet  

• If a toddler crawls towards a mirror at a rate of 0.25 m/s, then at what speed will the toddler and the toddler's image approach each other?

Answer : 0.25 m/s

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The image of an object in a plane mirror

(a) Same size as object(b) Laterally inverted(c) virtual(d) As far behind the mirror

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CONVEX MIRROR

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CURVED MIRROR If a concave mirror is thought of as being a slice of a sphere, then there would be a line passing through the center of the sphere and attaching to the mirror in the exact center of the mirror. This line is known as the principal axis. The point in the center of sphere from which the mirror was sliced is known as the center of curvature and is denoted by the letter C in the diagram below. The point on the mirror's surface where the principal axis meets the mirror is known as the vertex and is denoted by the letter A in the diagram below. The vertex is the geometric center of the mirror. Midway between the vertex and the center of curvature is a point known as the focal point; the focal point is denoted by the letter F in the diagram below. The distance from the vertex to the center of curvature is known as the radius of curvature (abbreviated by "R"). The radius of curvature is the radius of the sphere from which the mirror was cut. Finally, the distance from the mirror to the focal point is known as the focal length (abbreviated by "f"). Since the focal point is the midpoint of the line segment adjoining the vertex and the center of curvature, the focal length would be one-half the radius of curvature.

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Ray diagrams of convex and concave mirrorTwo rules of reflection for concave mirrors. They are:

Any incident ray traveling parallel to the principal axis on the way to a concave mirror will pass through the focal point upon reflection. Any incident ray passing through the focal point on the way to a concave mirror will travel parallel to the principal axis upon reflection.

The revised rules can be stated as follows:

Any incident ray traveling parallel to the principal axis on the way to a convex mirror will reflect in a manner that its extension will pass through the focal point. Any incident ray traveling towards a convex mirror such that its extension passes through the focal point will reflect and travel parallel to the principal axis.

PC F

principal axis

Concave mirror

P CF

Convex mirror

Any incident ray traveling parallel to the principal axis on the way to a concave mirror will pass through the focal point upon reflection.

Any incident ray traveling parallel to the principal axis on the way to a convex mirror will reflect in a manner that its extension will pass through the focal point.

PC F

Any incident ray traveling towards a convex mirror such that its extension passes through the focal point will reflect and travel parallel to the principal axis.

P CF

Any incident ray passing through the focal point on the way to a concave mirror will travel parallel to the principal axis upon reflection.

Concave mirror Convex mirror

PC F

A line through the centre of curvature, C from the top of the object

Concave mirror Convex mirror

PCF

A line through the centre of curvature, C from the top of the object

1. U < f

I

Characteristics:• Virtual• Upright• magnified

Applications :Shaving mirror

2. U = f

Characteristics:• Virtual• Upright• magnified

Applications :Sport light

3. f < U < 2f

I

Characteristics:• real• inverted• magnified

Applications :Projector

I

4. U = 2f or at C

Characteristics:• real• inverted• same size

Applications :Reflector in the projector

I

5. U > 2f or behind C

Characteristics:• real• inverted• diminished

Applications :telescope

PF

6. Infinity object

C I

Characteristics:• virtual• inverted• diminished

P CF

O IF

4. U > f

Characteristics:• virtual• upright• diminished

P CF

O IF

4. U < f

Characteristics:• virtual• Upright• diminished

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APLICATION OF REFLECTION OF LIGHT

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Rear view mirror

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Dentist mirror

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Periscope

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Activity

•2 plane mirror

•1 manila card

•1 scissors

•1 tape

•1 candle

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Prosedure