Light Waves

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SEEING THE LIGHT:

Luminous objects - those that emit their own light E.G. Sun, stars, fire, light bulbs

Non-luminous objects - those that don't emit light

We see non luminous objects because they reflect light

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REFLECTION:

When ray of light strikes plane mirror, reflection in such a way that angle of incidence = angle of reflection

Mirrors used to change direction of light

Simple periscope uses 2 mirrors

Rays from object strike mirror 1 at 45 degrees to normal to turned through 90 degrees by mirror

Strike mirror 2 at 45 degrees so turned through 90 degrees by mirror to observer

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IMAGES CREATED BY A PLANE MIRROR:

Images created on mirror created by rays of light from objects inside room striking mirror being reflected into eyes

Rays of light normally travel in straight lines, brain interprets rays coming from I

Real images - images created with rays of light actually passing through them E.G. cinema screen

Properties of Image in Plane Mirror:

Same distance behind mirror as object in front

Image and object same size

Image is virtual - rays of light don't actually pass through them and can't be produced on screen

Image laterally inverted - left and right side of image look interchanged

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REFRACTION:

Change in direction of a ray due to a change in speed because of a change in media

Ray can slow down or speed up as it crosses boundary between 2 media

G to A refract away, A to G refract to me

If ray strikes boundary between 2 media at 90 degrees, continues without change of direction

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REFRACTIVE INDEX:

Ratio between sine of angle of incidence and sine of angle of refraction

n = sin i / sin r

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Total Internal Reflection

As i in more dense medium increases, r also increases

At critical angle, angle of refraction is 90 degrees

Critical angle for glass is 42 degrees, water 49 degrees

Therefore, critical angle:

At it, angle of refraction is 90 degrees

The angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted but totally reflected.

Critical angle for particular medium related to refractive index by:

sin c = 1 / n

Total internal reflection - the complete reflection of a light ray reaching an interface with a less dense medium when the angle of incidence exceeds the critical angle.

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USING TOTAL INTERNAL REFLECTION:

1. Prismatic periscope

Light enters prism ABC at 90 degrees and passes through surface of AB

Strikes surface of AC at 45 degrees

Critical angle of glass 42 degrees to ray totally internally reflected and turned through 90 degrees

Ray travels to second prism and again totally internally reflected

2. Reflectors

Light entering prism undergoes TIR twice

Arrangement used in bicycle reflectors and binoculars

3. Optical Fibres

Composed of central core of optically dense glass with high refractive index cladded or coated with optically less dense glass

Fibres very narrow so light entering strikes boundary of glasses at angle greater than critical angle

Light totally internally reflected

Used for transmitting light, communications, endoscopes

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DISPERSION:

When white light passes through prism, emerges as band of colours - spectrum