Light Rays

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In this presentation you will: explore the reflection and refraction

of light rays

Light Rays

They might pass through an object,

Light travels in straight lines called rays. Light rays behave in different ways when they encounter objects.

They might become absorbed or partially absorbed by objects,

They might be reflected by non-transparent surfaces.

In this presentation you will explore what happens to light rays when they hit a mirror or travel through a transparent solid. You will then see how images appear when light rays travel through lenses.

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Light Rays

We see stars (including the Sun!) because they emit light.

Objects are seen by the eye either because they reflect light or because they emit light.

Reflections

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We see the Moon because it reflects light from the Sun.

Light Rays

A light ray that hits a mirror is calledthe ray of incidence (or incident ray).

The light ray that bounces off the mirror is called the ray of reflection (or reflected ray).

A mirror will reflect rays of light.

Reflections

The normal is a line perpendicular to the mirror.

The angle of incidence from the normal is represented by i.

The angle of reflection from the normal is represented by r.

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Incident rayReflected

rayNormal

ir

Light Rays

Parallel light rays hitting a very smooth reflective surface will all be reflected through the same angle.

The Law of Reflection

This is called specular reflection.

Reflected light obeys the law of reflection,

“the angle of reflection equals the angle of incidence”.

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Light Rays

The reflection of light from rough surfaces is called diffuse reflection.

The Law of Reflection

Such surfaces consist of a large number of different reflecting planes, each one producing specular reflection.

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Light Rays

1

Which light ray is called the incident ray?

Question

A) The light ray that hits the mirror.

B) The light ray that bounces off the mirror.

C) The line perpendicular to the face of the mirror.

Light Rays

2

For reflection, the angle of incidence is:

Question

A) at 90° to the face of the mirror.

B) the same as the angle of reflection.

C) twice the angle of reflection.

Light Rays

When a ray of light (incident ray) travels from one transparent material or medium to another, part of the ray is reflected (reflected ray) at its boundary and part enters the second piece of material.

Reflection and Refraction

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The change in direction of the ray of light, entering the second piece of material, is due to light changing speed. This is known as refraction.

The ray entering the second piece of material is known as the refracted ray.

Normal

Incident ray Reflected ray

Air

Glass

Refracted ray

Light Rays

Refraction

Therefore, when light travels from water into air, it speeds up. This is shown in the diagram above. Each line represents a wave front moving from one medium to another.

If the light is traveling at right angles to the boundary, it passes through the boundary with no deviation.

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For example, light travels slower through water than through air.

Water (lower speed) Air (higher speed)

Wavelength has increaseddue to increased speed

Direction ofmovement

boundary

Light Rays

Refraction

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To explain this, it is necessary to again think of light as a wave front, moving from one medium to another.

However, when the light is not traveling at right angles to the boundary, the direction of the light changes.

As the first part of the wave enters the boundary, it speeds up. The rest of the wave continues at the same speed.

New direction

Air

Water

The result is that the wave front progresses further on one side than the other, causing the wave front to swerve.

Light Rays

A simple experiment can be performed to see refraction. This involves passing a ray of light at an angle into a glass block.

Refraction in a Glass Block

This causes the ray to bend toward the normal.

The light then returns to its original speed when it leaves the glass.

This causes the ray to bend away from the normal.

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When the ray, traveling in air, enters the glass, it slows down.

Light Rays

The refractive index is defined as the ratio of the velocity of light in a vacuum to the velocity of light in the medium:

Refractive Index

wheren = refractive indexc = velocity of light in vacuum in m/sv = velocity of light in the medium in m/s

The refractive index of a material has no units as it is a ratio. Note that it is always larger than 1, as the velocity of light in a medium can never exceed the velocity of light in a vacuum.

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n =c

v

Light Rays

Refractive Index

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The table shows some common substances and their refractive indices.

The lower the refractive index, the higher the speed of the light traveling through the material.

1.000293Air @ 0°C, 1atm

1.333Water

1.309Ice

1.52Glass (Crown)

2.419Diamond

Refractive IndexSubstance

Light Rays

Snell’s Law of Refraction

The equation relating these angles is given as:

Where n1 and n2 are the refractive indices for two different media.

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Willebrord Snell (1580-1626)discovered the relationship between the angle of incidence (i) and the angle of refraction (r) of light passing through two media.

REFRACTIVE INDEX n2

REFRACTIVE INDEX n1

Normal

Angle of incidence

Angle of refraction

i

r

n1 sin i = n2 sin r

Light Rays

3

Which of the following fractions gives the ratio of the refractive index of water to the refractive index of air?

Question

A) sin a / sin c

B) sin a / sin d

C) sin b / sin c

D) sin b / sin dWater

Air

Normal

ab

cd

Light Rays

However, if the incident ray comes from a more optically dense material to a less dense material (for example glass to air) at a critical angle, ic, then the light will refract at 90° to the normal, along the boundary between the two materials.

Critical Angle

The critical angle ic is given by:

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Note: this is only true when n2 > n1

As you have seen, a light ray will refract when passing at an angle through a boundary between two materials.

Incidentray

Normal

Refracted ray

i

r

Incidentray

Refracted ray

ic

sin ic= = n1

n2

refractive index of air

refractive index of glass

Light Rays

This is called Total Internal Reflection.

If the incident ray exceeds the critical angle, then all of the light will be internally reflected back from the boundary region into the first material. The boundary region acts as a mirror.

Total Internal Reflection

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An interesting application where total internal reflection is used, is in fiber optics.

Incidentray

i

Refracted ray

Light Rays

4

If the critical angle between glass and air is 41°, what would happen to a light ray, traveling in glass, that hits the glass-air boundary at an angle of incidence of 50°?

Question

A) Some of the light would be refracted.

B) The light would come out along the glass-air boundary.

C) All the light would be internally reflected.

Light Rays

Since the refractive index of a transparent material, such as glass, is different for each color, the angle of refraction will depend on the color of the light ray entering the material.

Sunlight is often called white light, since it is a combination of all the visible colors.

Dispersion

If a piece of glass has parallel sides, the light will return in the same direction that it entered the material.

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Light Rays

If the material is shaped like a prism, the angles for each color will be exaggerated, and the colors will be displayed as a spectrum of light.

Dispersion

This breaking up of white light into its constituent colors is called dispersion.

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Light Rays

A converging lens refracts light rays passing through it, parallel to the principal axis, to a point called the focus or focal point.

A converging or convex lens is thicker in the middle than at the edges.

Converging Lens

The distance of the focal point from the center of the lens is called the focal length of the lens.

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Principal axis

Focal point

Focal length

Light Rays

Any light emitted from the focal point is refracted by the lens to become parallel to the principal axis.

The fact that a convex lens is thicker across its middle is an indicator that it will converge rays of light which travel parallel to its principal axis.

Converging Lens

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Light Rays

The focal point of a diverging lens is found by extending the refracted rays backwards behind the lens, to a point where the refracted rays intersect.

A diverging or concave lens is a lens that is thinner in the middle than at the edges, causing rays traveling parallel to its principal axis to diverge.

Diverging Lens

As light doesn’t actually come from this point it is called a virtual focus.

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A diverging lens is said to have a negative focal length.

Principal axis

Principal point

Focal point

Focal length

Light Rays

5

Which of the diagrams shows the correct ray paths for a light beam passing through a convex lens?

Question

A)   B)  

C)   D)  

Light Rays

show knowledge and understanding of the reflection of light rays.

show knowledge and understanding of the refraction of light rays.

After completing this presentation you should be able to:

show knowledge and understanding of how lenses use refraction.

Summary

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