LIGHT

Light : What is Light?

Light carries energy and travels as a wave.

Light travels at 300 000 000 m/s or 300 000 km/s (much faster than sound).

Light waves travel in straight lines.

Light waves undergo reflection, refraction diffraction and interference.

Light waves are transverse

Reflection

Reflection

• Laws of Reflection

– The angle of incidence equals the angle of reflection [ <i = < r ]

- The incident and reflected rays and the normal lie in the same plane.

Reflection: the bouncing back of light as it strikes a barrier (mirror).

Definitions:

1. Incident ray (i): the ray of light that strikes the mirror (barrier)

2. Normal (N): a line drawn perpendicular to the mirror drawn at the point where the incident ray strikes the mirror.

3. Angle of incidence (<i): the angle between the incident ray and Normal.

4. Reflected ray (r): the ray of light leaving the mirror surface.

5. Angle of reflection (<r): the angle between the reflected ray and the Normal.

1. Draw a line and label it ‘mirror’.

2. Draw a dot on top of the line and label it as ‘O’

3. Draw a line (incident ray) from the ‘O’ to the mirror and label it i1.

4. Draw a ‘Normal’ perpendicular to the point where i1 meets the mirror and label it N1.

5. Use a protractor to measure the angle of incidence (<i1).

6. Use the formula: angle of incidence = angle of reflection, and draw the reflected ray (r1).

7. Label the angle of reflection as <r1

Reflection: Image location by construction

8. Repeat the steps 3 to 7 for i2, <i2, N2, <r2 and r2.

9. Extend r1 and r2 backwards as dotted lines. They meet at the Image location.

10. Measure the distance from the object to the mirror.

11. Measure the distance from the image to the mirror.

(draw diagram on the board)

Question:

What is your conclusion?

CONCLUSION

When a plane mirror forms an image:

1. The image is the same size as the object.

2. The image is as far behind the mirror as the object is in front.

3. A line joining equivalent points on the object and image passes through the mirror at right angles.

REFRACTION

Refraction : Bending light

The speed of light waves depends on the material they are travelling through.

If the light waves enter a different material [e.g. from glass into air] the speed changes.

This causes the light to bend [or refract].

Air = Fastest Diamond = slowestGlass = slower

Glass

Refraction: the bending of light as it moves from one medium to another due to

change in wave speed.

Refraction

• In both cases the speed of the wave has decreased. This is indicated by the decrease in wavelength!

Refraction : Investigating Refraction

Angle i

Angle r

Definitions:

1. Incident ray (i): the ray of light that strikes the boundary.

2. Normal (N): a line drawn perpendicular to the boundary drawn at the point where the incident ray strikes the boundary.

3. Angle of incidence (<i): the angle between the incident ray and Normal.

4. Refracted ray (r): the ray of light in the other medium.

5. Angle of refraction (<r): the angle between the refracted ray and the Normal.

Speed of light

• v is the speed of light in the new medium.

• c= 3.0 x 108 m/s

• As the index increases the speed decrease.

n is the absolute index of refraction, Refractive index. This is a measure of optical density. n is defined as the ratio of the speed of light in a vacuum to the speed of light in a new medium.

Refractive indexMedium Refractive index

Diamond 2.42

Glass (crown) 1.52

Acrylic plastic (Perspex) 1.49

Water 1.33

Exercise:Calculate the speed of light in;(a) Diamond(b) Glass (crown)(c) Acrylic plastic (Perspex)(d) Water

-As light moves from one medium to another, it undergoes refraction.-The refractive index of a denser medium is greater than that of a less dense medium.-When light moves from a denser medium to a less dense medium, it bends away from the normal. -When light moves from a less dense medium to a more dense medium, it bends towards the normal.

SNELL’S LAW

• In 1620, Willebrord Snell the Dutch scientist discovered the link between the two angles: their sines are always in proportion.

• When light passes from one medium into another:

sin i is a constant

sin r

That is:

refractive index = sin i

sin r

Example

1. A ray of light strikes a material at 35o and is refracted at 15o. What is the refractive index of the material?

2. In an experiment, it was discovered that light travels at 1.9 x 1o8m/s in a certain material. Calculate

a. The refractive index of the material

b. The angle of refraction given the angle of incidence as 38o.

Exercise

1. In an experiment, it was discovered that light travels at 1.7 x 1o8m/s in a certain material. Calculate

a. The refractive index of the material

b. The angle of refraction given the angle of incidence as 38o.

2. Calculate the angle of refraction if light (in air) strikes water at an angle of incidence of 23o.

3. What is the angle of incidence when light is refracted at 13o in a Diamond block?

Refraction: Determining the refractive index of glass

1. Place a glass block on an A4 paper and draw around it.

2. Place 2 pins on one side of the glass block and label them p1 and p2.

3. Place 2 more pins on the other side of the block such that they make a straight line with the images of p1 and p2 in the block. Label them p3 and p4.

4. Remove the glass block and the pins.5. Draw a line passing through p1 and p2

up to the upper bounder of the block.6. Draw another line passing through p4

and p3 up to the lower boundary of the block.

7. Join the upper line to the lower line.8. From the upper line, draw a normal and

measure the angles of incidence and refraction.

9. Calculate the refractive index of the glass block.

x p1

x p2

x p3

x p4

Refraction : Summary

When light bends this is called refraction.

Refraction happens because the light changes speed [or velocity].

When light enters a more dense medium [e.g. glass], it bends towards the normal.

When light enters a less dense medium [e.g. air], it bends away from the normal.

If the incident ray hits a surface at 0º (light passing throughthe normal), no refraction occurs.

Remember that the angle of reflection [r] and angle of refraction [r] have the same symbol.

In reflection, <i = <r

In refraction, <i <r

Consequences of Refraction of light1. Apparent depth

R – Real Depth

A – Apparent Depth

If the chest is 20m below the surface, calculate its apparent depth.

If the chest is 20m below the surface, calculate its apparent depth.

Dispersion of light using a glass prism

• A prism is a triangular block of glass or plastic.

• The sides of a prism are not parallel.

• When light is refracted by a prism, it comes out in a different direction. It is Deviated.

• If a narrow beam of white light is passed through a prism, it splits into a range of colours called a Spectrum. This effect is called Dispersion of light.

• The spectrum of white light is a continuous change of colour from beginning to end.

• The spectrum has Red, Orange, Yellow, Green, Blue, Indigo, Violet (ROY-G-BIV)

• In the white light spectrum,

– Red light is deviated (bent off-course) the least. It is refracted the least.

– Violet is refracted the most.