Lesson 6

• A village in Italy does not receive sunlight from November 11th – February 2nd because it is in a valley.

The Use of Mirrors• In 2006 they spent 170,000 $ on a 5m x

8m mirror that reflects light down to the village in the winter.

• The mirror is controlled by computers that follow the Sun across the sky

The science of how light reflects and bends is called geometric optics.

The Law of Reflection

Smooth, shiny surfaces like calm water, mirrors, glass, and even polished metal allow you to see an image.

The smoother the surface is, the better the image will be. An image is a reproduction of an

object produced by an optical device like a mirror.

An optical device is any technology that uses light

A ray diagram demonstrating the law of reflection Normal – Dashed line drawn

perpendicular to the mirror at point of reflection.

Incident Ray – The Incoming Ray of light

Reflected Ray – The outgoing Ray of light

A ray diagram demonstrating the law of reflection Angle of Incidence – The angle

between the incident ray and the normal. (i)

Angle of Reflection – The angle between the normal and the reflected ray. (r)

Same as incident ray

Law of Reflection

When light reflects off a surface, the angle of incidence is always equal to the angle of reflection.

Using the Law of Reflection The law of reflection can be written

using mathematical symbols. The Greek letter theta, θ, is

commonly used as the symbol for an angle

Subscripts are used to identify the angle. Angle of incidence = θ i Angle of reflection = θ r

Using the Law of Reflection Law of reflection is: θi = θr The angle

of incidence and the angle of reflection are always measured from the normal and not from the surface of the object. This is because some surfaces are curved, making it difficult to measure an angle from the surface.

Using the Law of Reflection The law of reflection is widely used in

many everyday use items such as reflectors

Two plane mirrors placed at a of 90° to each other can reflect light back to its source from a wide variety of angles.

In each reflection the angle of incidence is equal to that of the angle of reflection

Plane Mirrors

The law of reflection applies to light rays reflected from both flat mirrors and curved mirrors.

Plane Mirror - Any mirror that has a flat reflective surface

The image in a plane mirror appears to come from behind the mirror. It is not possible to catch this image on a piece of paper placed behind the mirror, since no light from the object reaches this point.

1. Consider the diagram at the right. Which one of the angles (A, B, C, or D) is the angle of incidence? ______ Which one of the angles is the angle of reflection? ______

B

C

2. A ray of light is incident towards a plane mirror at an angle of 30 degrees with the mirror surface. What will be the angle of reflection?

90 – 30 = 60 degrees

Law of reflection states that angle of incidence = angle of reflectionTherefore, the angle of reflection will be 60 degrees

3. Perhaps you have observed the image of the sun in the windows of distant buildings near the time that the sun is rising or setting. However, the image of the sun is not seen in the windows of distant building during midday. Use the diagram below to explain, drawing appropriate light rays on the diagram.

4. A ray of light is approaching a set of three mirrors as shown in the diagram. The light ray is approaching the first mirror at an angle of 45-degrees with the mirror surface. Trace the path of the light ray as it bounces off the mirror. Continue tracing the ray until it finally exits from the mirror system. How many times will the ray reflect before it finally exits?

Plane Mirrors

Because the light rays are not coming from where your image appears to be, it is called a virtual image. Virtual image - any image formed

by rays that do not actually pass through the location of the image

Real Images can be projected on to a screen and touched.

Image Orientation in a Plane Mirror The law of reflection also applies

to looking at objects in a mirror. The image in a plane mirror is an

exact reflection of the actual image. To view an image of a object in a

mirror, you must sight along a line at the image location. As you sight at the image, light travels to your eye along the path shown in the diagram below.

Image Orientation in a Plane Mirror The diagram shows that the light

reflects off the mirror in such a manner that the angle of incidence is equal to the angle of reflection.

In order to see the image, incident rays from the object must bounce off the mirror at the same angle as the reflected rays. If the eye does not sight on the point of incidence on the mirror the image will not be visible.

When the image is sighted, it is visible that the drawing an extended line of the reflected ray through the back of the mirror will lead to the image of the object.

Plane Mirror drawings

Images are always drawn in dotted lines

the point at which the extended reflected rays converge (come together) behind the mirror indicates the size and distance of the object

Note: Converge - come together

Diverge - spread apart

to locate any point on the image, you need to draw only two light rays

a ray from the top of the object parallel to the base line and a ray drawn from the top of the object to the point where the base line meets the mirror

Plane mirror examples

Plane Mirror Lab

Questions

1. What is the difference between a reflected ray and an incident ray? I (1)

2. (a) What does the law of reflection state in words? C (1)

(b) Write the law of reflection using mathematical symbols. C(1)

3. What is a plane mirror? I (1) 4. What is a virtual image? I (1) 5. Draw the image of the letter B and N

in a plane mirror drawing. C (2)