Reflection of light

Law of Reflection

The angle of incidence (i) is equal to the angle of reflection (r)

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Note: Both angles are measured with respect to the ‘normal’. This is a construction line that is perpendicular to the reflecting surface at the point of incidence.

The image formed by a plane mirror

The image produced by the plane mirror is:

• The same size as the object
• The same distance behind the mirror as the object is in front
• Upright (the same way up as the object)
• Back-to-front compared with the object (lateral inversion)
• Virtual

Virtual images

VIRTUAL images are formed where light rays only appear to come from. A virtual image cannot be cast onto a screen.

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The image formed by a projector is known as a REAL image because light rays travel to it.

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Choose appropriate words to fill in the gaps below:

The law of reflection states that the angle of __________ is always _______ to the angle of incidence.

Both angles are measured relative to the _________, a line that is at _______ degrees to the reflecting surface at the point of reflection.

A plane mirror forms a _______ image which is unlike a _____ image in that it cannot be cast onto a screen. The image in a plane mirror is also the same ______ and the same way up as the object.

real

virtual

reflection

size

normal

equal

ninety

WORD SELECTION:

real

virtual

reflection

size

normal

equal

ninety

Light Refraction

Refraction occurs when a wave changes speed as it passes from one region to another.

This speed change usually causes the wave to change direction.

Water waves slow down as they pass over from a deeper to a shallower region.

Light slows down as it passes from air into glass, perspex or water.

Refraction experiment

Typical results:

No deviation occurs when the angle of incidence is zero.

Increasing the angle of incidence increases the deviation.

Refraction of light at a plane surface

(a) Less to more optical dense transition (e.g. air to glass)

Light bends TOWARDS the normal.

The angle of refraction is LESS than the angle of incidence.

(b) More to less optical dense transition (e.g. water to air)

Light bends AWAY FROM the normal.

The angle of refraction is GREATER than the angle of incidence.

Why a pool appears shallow

Complete the paths of the RED light rays:

Dispersion

Dispersion occurs when a prism splits the colours of white light into the spectrum.

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This occurs because the refractive index of the glass or perspex of the prism varies with the colours of the spectrum that make up white light.

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Violet has the greatest refractive index and therefore deviates the most.

Red has the lowest and deviates the least.

Choose appropriate words to fill in the gaps below:

Refraction occurs when a wave changes ______ as it crosses the boundary between two regions. The _________ of the wave also usually changes.

Light rays deviate ________ the normal when they pass from less dense air to more dense _________. The greater the angle of incidence the greater is the _________.

Different ______ of light deviate by different amounts. Violet deviates the _____. A prism can be used to split the colours of white light into a spectrum. This is called _________.

most

speed

direction

deviation

colours

perspex

towards

WORD SELECTION:

dispersion

most

speed

direction

deviation

colours

perspex

towards

dispersion

Diffraction of light

Diffraction occurs when waves spread out after passing through a gap or round an obstacle.

Diffraction becomes more significant when the size of the gap or obstacle is reduced compared with the wavelength of the wave.

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The wavelength of light is about 0.0005mm.

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Therefore light diffraction is only noticeable with very small apertures or where an image is highly magnified.