Optics 光學1. Reflection of Light

Reflection ( 反射 )

Light raysLight is a source of energy.

We detect it with eyes.

Experiment : Reflection by plane mirrors

• Aims:

• Study the reflection of light rays by a plane mirror and also find out how to locate the image formed by a plane mirror.

Experiment 1.1 Law of Reflection

1. Direct a ray of light at a plane mirror as shown.

Ray-box

Plane mirror

normal

Experiment 1.1 Law of Reflection

1. Direct a ray of light at a plane mirror as shown.

Angle of incidence ( 入射角 ) i of the light ray

Angle of reflection ( 反射角 ) r

Experiment 1.1 Law of Reflection

1. Direct a ray of light at a plane mirror as shown.

2. Measure the angle of incidence and the angle of reflection.

Experiment 1.1 Law of Reflection

1. Direct a ray of light at a plane mirror as shown.

2. Measure the angle of incidence and the angle of reflection.

3. Repeat several times by changing the anlge of incidence.

3. Repeat by changing the angle of incidence. Record the results.

Angle of incidence i 0 10 20 30 40 50

Angle of reflection r            

How is the angle of incidence related to the angle of reflection?

___________________________________________________________

How is the angle of incidence related to the angle of reflection?

The angle of reflection is equal to the angle of incidence.

Laws of reflection :

There are two laws of reflection

1. The incident ray, the reflected ray and the normal all lie in the same plane. This means that all three can be drawn on a flat sheet of paper

2. The angle of incident (i) is equal to the angle of reflection (r). The ray is reflected from the mirror at the same angle as it arrives.

Example 1.

(b) Complete the following ray diagram by drawing all the reflected rays.

50°

Plane mirror

Plane mirror

(a) What is the angle of incidence?

(c) What is the relation between the original incident ray and final reflected ray

The angle of incidence = 40°

40°

40°50°

50°

50°

They are parallel.

Arrange light rays to hit the plane mirror as shown in figure. Study how the rays are reflected. Look at the rays from above. Also bend down and look along the rays

The reflected rays seem to come from a point __________ (in front of / behind) the mirror.

behind

Conclusion: 1. In reflection, the angle of incidence is equal to the angle of _________________. 2. An image formed by a plane mirror is as far behind the mirror as the object is ____________.  3. The line joining the object and the image is _____________ to the mirror.

reflection

perpendicular

abc

Laws of reflection

Object Image

Object Image

Object Image

Object

Complete the rays and located the image

Image

Man

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

ImageEye

Man

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

ImageEye

Man

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

ImageEye

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

Image

1.8m

0.9m

There are two similar triangles

(a) Minimum length = 0.9m

Man

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

ImageEye

Man

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

ImageEye

Man

A man 1.8 m tall stands in front of a plane mirror. His eyes are 1.7 mAbove the ground. (a) What is the minimum length of a mirror that allows him to see his

whole body?(b) At what height should the mirror be hung?

Eye

There are two similar triangles

1.7m

0.85m

(b) It should 0.85 above the ground

Object

Uses of plane mirrors

1. To help use look at ourselves,2. To make a room look bigger,3. In periscopes