LET'S LEARN ABOUT
LIGHT
MADRASAH MU’ALLIMAAT MUHAMMADIYAH YOGYAKARTA
2024
Use the of reflection of light to explain how an image is formed in a plane mirror
IN THIS CApTHER YOU WILL :
1
2
3
Construct ray diagram for reflection
Investigate the refraction of ligt
Draw ray diagram to show how lenses from images
Describe the difference between real and virtual image
Describe total internal reflection and how it is used
Describe how the visible spectrum is formed
4
5
6
7
LIGHT
13.1 Refelction of light
Light usually travels in straight lines. It changes direction if it hits a shiny surface. This change in direction at a shiny surface such a mirror is called reflection.
Ray Box
An apparatus used produces a ray (narrow beam) of light
Laser light
13.1 Refelction of light
What happens when we stand in front of a mirror?
13.1 Refelction of light
What happens when we stand in front of a mirror?
“when you look in a mirror, rays of light from face reflect off the shiny surface and back to your eyes”
13.1 Refelction of light
Ray Diagram
13.1 Refelction of light
Incident ray: a ray of light arriving at a surface
Reflected ray: a ray of light which has been reflected from a surface
Normal: the line drawn at right angles to surface at the point where a ray hits to the surface
Angle of incidence: the angle between the incident ray and normal drawn at the point where the ray hits the surface
Angle of reflection: the angle between the reflected ray and normal drawn at the point where the ray hits the surface
13.1 Refelction of light
angle of incidence = angle of reflection
i = r
Notes: The angles are measured between the rays and the normal
13.1 Refelction of light
13.1 Refelction of light
13.1 Refelction of light
13.1 Refelction of light
13.1 Refelction of light
13.2 Refraction of light
Refraction occurs when a ray of light travels from one medium to another
For example
If you look at the bottom of a swimming pool, you may see patterns of shadowy ripples
13.2 Refraction of light
The straw is partly immersed in the drink. Because of the refraction of the light coming from the part of straw that is underwater, the straw appears bent
13.2 Refraction of light
In the refraction, the ray of light may change direction. Refraction happens at boundary between the two materials. The ray approaching the boundary is called incident ray and the ray leaving boundary is called the refracted ray. The angle of incident, i, and angle of refraction, r, are measured to the normal drawn at the point where the ray hits the boundary
13.2 Refraction of light
Demonstration the refraction of a ray of light when it passes through a rectangular block of glass. The ray bends as it enters the block. As it leaves, it bends back to original direction
Changing Direction
13.2 Refraction of light
You can see that the direction in which the ray bends depands on whether it is entering or leaving the glass.
Changing Direction
Air
(less dense)
Glass
(denser)
Water
(denser)
13.2 Refraction of light
A ray of light may strike a surface at a angle of incidence of 0 ̊ , as shown in figure above. In this case, it does not bend - is simply passes straight through and carries on in the same direction
Changing Direction
13.2 Refraction of light
A ray of light may strike a surface at a angle of incidence of 0 ̊ , as shown in figure above. In this case, it does not bend - is simply passes straight through and carries on in the same direction
Changing Direction
13.2 Refraction of light
Why is light refracted?
Light is refracted because it travels at different speeds in different material
Picture a truck’s wheels slipping off the road into the sand. The truck turns to the side because it cannot move so quickly through the sand
13.2 Refraction of light
Why the swimming pool appears to be shallower than it is?
EXPLANATION
13.2 Refraction of light
Refractive Index
( n )
The refractive index of a material measures how much light is bent
Formula of refractive index
or
13.3 Total Internal Reflection
Total Internal Reflection
When a ray of ligt strikes the inner surface of a material and 100% of the light reflects inside it
Light is refracted when it travels from one medium to another
However, sometimes not all light is refracted. Some could the reflected from the surface
Increase the incidence angle until it equals the critical angle
Angle of incidence is small. Only faint reflected ray. Refracted ray bends away from normal
Angle of incidence increase, more light is reflected inside the block. Refracted ray bends away further from normal
Refracted ray emerges along and parallel to the surface of the block. For a particular angle of incidence. This angle is called the critical angle
Angle of incidence is greater than the critical angle. All the light is reflected inside the block. No refracted ray emerges from the block
13.3 Total Internal Reflection
If the angle incidence is greater than the critical angle, the light is entirely reflected inside the glass
This is known as total internal reflection
Total | 100 % of the light is reflected |
Internal | It happens inside the glass |
Reflection | The ray inentirely reflected |
13.3 Total Internal Reflection
Critical angle =
Definition the minimum angle of incidence at which total internal reflection occurs
1. Critical angle depends on the material through which a ray is travelling
2. The greater the refractive index amaterial, the smaller the critical angle and vice versa
13.3 Total Internal Reflection
Critical angle formula =
Work Example:
Find the critical angle, c, for diamond. Assume that refractive index n = 2,40
13.3 Total Internal Reflection
Critical angle application - Telecomunication
13.3 Total Internal Reflection
Critical angle application - Medicine
13.3 Total Internal Reflection
13.4 Lenses
LENSES
13.4 Lenses
Lenses can be devided into two types, according to their effect on light
Converging Lens
Diverging Lens
Fatter in middle than at edges
Thinner in middle than at edges
13.4 Lenses
Converging Lens
A lens that causes rays of light parallel to the axits to converge at the principal focus
Diverging Lens
A lens that causes rays of light parallel to the axits to diverge from the principal focus
13.4 Lenses
Converging Lens
After the parallel rays pass through the lens, they converge on a single point. The principal focus or focal point
After they have passed through the principal focus, they spread out again
13.4 Lenses
Converging Lens
An additional converging lens can be used to produce a beam of parallel rays
13.4 Lenses
Converging Lens
The fatter the lens, the closer the principal focus is to the lens. Fatter lens has shorter focal length. See diagram below
High-temperature objects give off hot light or incandescent light. For example, the Sun is a hot yellow star that warms the Earth and fills the world with incandescent light.
Important definition to describe image formed
Enlarged: used to describe an image which is bigger than the object
Diminished: used to describe an image which is smaller than the object
Inverted: used to describe an image which is upside down compared to the object
Upright: used to describe an image which is the same way up as the object
13.4 Lenses
High-temperature objects give off hot light or incandescent light. For example, the Sun is a hot yellow star that warms the Earth and fills the world with incandescent light.
Worked example
Draw a ray diagram to find the image formed of a 3 cm tall object placed 12 cm from a converging lens which a focal length of 5 cm
13.4 Lenses
F
F
Application of lens (1) – Magnifying glass
13.4 Lenses
Magnifying glass is a converging lens. User should hold it close to a small object to see a magnified image
Application of lens (1) – Magnifying glass
13.4 Lenses
The object viewed by a magnifying glass should be closer to the lens than the principal focus. Ray diagram for a magnifying glass is as followed
Application of lens (2) – Correct eyesight problem
13.4 Lenses
The object viewed by a magnifying glass should be closer to the lens than the principal focus. Ray diagram for a magnifying glass is as followed
Application of lens (2) – short sight
13.4 Lenses
A person with short sight can see close up objects clearly, but cannot form a clear image of distant objects. The images formed in front of the retina
To correst this, a deverging lens is used to make rays from the distant object divergen
Application of lens (2) – long sight
13.4 Lenses
A long sighted eye can focus the distant objects but not close objects
Two reasons: a) They eyeball is to short b) The lens cannot become strong enough to the rays from a close object cannot be converged enough to form an image on retina
To correct this, a converging lens is used to make rays from the distant object converge
13.5 Dispersion
Dispersion of light
When white light passes through glass, it refracts as it enters and leaves the glass, and can be split into a spectrum of coulors
Definition : The separation of different wavelengths of light because they are refracted through different angles