VISIBLE LIGHT

Reflection, refraction and diffraction of light, diagrams and explanations

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

This page will answer many questions e.g.

How do draw ray diagrams?

What is refraction?

What is diffraction?

How do explain why waves refract or diffract?



  • Know and understand that light waves can be reflected, refracted and diffracted.

  • Light waves are readily reflected off smooth flat surfaces e.g. light reflected off a smooth surface like a mirror.

    • Reflection

      • The vertical dotted line is called the 'normal', it isn't a ray, but helps in the construction and interpretation of ray diagrams. A plane mirror means one of a perfectly flat surface.

      • Angle 1 = angle of incidence of incident ray. Angle 4 = angle of reflection of the reflected ray.

      • Angle 2 = Angle 3. All angles are measured with respect to the 'normal' which is at 90o to the surface.

    • Refraction: The bending of the light ray at an interface between two media of different density.

    • Know that light waves are not refracted if travelling along the normal (diagram 1 below).

      • 1. No refraction when a light ray strikes a different medium at 90o to the surface ie 'down' the normal.

        • The same applies to 3 and 4 for the central ray in the diagram.

      • 2. Double refraction through a rectangular glass block at the air/glass interfaces, note that when the ray emerges back into air its path is parallel to the original incident ray.

      • 3. Refraction of two rays at the two surfaces of a diverging concave lens.

      • 4. Refraction of two rays at two of the surfaces of a triangular glass or plastic prism.

      • 5. Refraction of two rays at the two surfaces of converging concave lens.


REFLECTION

  • Know that light waves are reflected and refracted at boundaries between different materials.

  • Apart from luminous objects (give out their own light), we see objects by reflected light.

  • If the light is reflected from a very smooth 'shiny' surface we see a mirror image eg a silvered mirror, aluminium foil, shop window etc.

  • If the surface is uneven, the light is scattered in all directions eg you don't see a mirror image looking at tissue paper or a sliced section of apple.

  • a) Know that the normal is a construction line perpendicular (at 90o) to the smooth reflecting surface at the point of incidence.

  • b) Know that the angle of incidence is equal to the angle of reflection.

  • c) Know that the image produced in a plane mirror is virtual, upright and laterally inverted.

    • You will be will be expected to be able to construct ray diagrams to match these situations.

    • the vertical dotted line is the normal at 90o to the reflecting surface

    • reflection is the simple 'bouncing' off a plane smooth-shiny surface.

    • angle 2 = angle 3: angle of incidence i = angle of reflection r

    • The construction of the ray diagram to show the formation of a virtual image by a plane mirror and from which you can deduce the characteristics of the virtual image.

      • You are expected to be able to construct the diagram of the virtual image formed in a plane mirror.

    • The features of the virtual image formed by a plane mirror are ...

      • The image is the same size as the object.

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

      • The image is upright - the same way up as the object (if not it would be inverted, and you would look upside down!).

      • The image is virtual because the image appears to be behind the mirror.

      • The image is laterally inverted, the 'left' of the object now appears on the 'right' side and the 'right' of the object appears to be the 'left' side of the image.

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REFRACTION

  • You should know and understand that waves undergo a change of direction when they are refracted at an interface.

    • Refraction diagram above.

      • the vertical .... lines are the two normal

      • angles 1 and 3 = angles of incidence

      • angles 2 and 4 = angles of refraction

      • note that when a ray enters a more dense medium (air ==> glass), the ray bends towards the normal, and on entering a less dense medium (glass ==> air) the ray bends away from the normal

      • there maybe a little reflection of incident rays 1 and 3, but most of the rays are refracted.

    • Explanation of refraction

    • The above diagram illustrates the phenomena of refraction by considering what happens to waves eg visible light.

      • You can think of the parallel lines as representing a series of crests of waves eg think waves on the sea or ripples in a pond on throwing a stone in.

    • Light rays passing from a less dense medium to a more dense transparent medium.

      • Note: Ray 2 refracted. You do get some reflection too, ray 1.

    • Refraction A: When waves from a less dense medium, hit a boundary interface, and enter a more dense medium, the waves 'bend towards the normal' ie refraction occurs.

      • This happens because on entering the more dense medium, the waves slow down causing the change wave in direction.

      • The obvious examples you see in optics experiments are light rays passing from air into more dense transparent plastic blocks or triangular and rectangular glass prisms.

    • Light rays passing from a more dense medium to a less dense transparent medium.

      • Note: Ray 2 refracted. You do get some reflection too, ray 1.

    • Refraction B: When waves from a more dense medium, hit a boundary interface, and enter a less dense medium, the waves 'bend away from the normal' ie refraction occurs.

      • This happens because on entering the less dense medium, the waves speed up causing the change in wave direction.

      • The obvious examples you see in optics experiments are light rays emerging from transparent plastic blocks or triangular and rectangular glass prisms, and passing out into less dense air.

      • When you observe an object half in water and half in air e.g. poking a stick into still water, you see a 'bent' distorted image, because, the light rays from the object and bent at the air-water interface because of refraction.

    • If the waves hit the interface at an angle of 90o (perpendicular) to the interface between the two mediums, there is still a change in speed and wavelength, but there is NO refraction and the wave frequency remains the same.

 


DIFFRACTION

  • Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves go round corners! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners! The effect is so small with light (tiny wavelength), you don't notice it, but you see water waves bending around objects at the seaside and you can hear sounds from round a corner.

  • You should appreciate that significant diffraction only occurs when the wavelength of the wave is of the same order of magnitude as the size of the gap or obstacle.

  • A: There is a relatively small diffraction effect when waves pass through a wide gap that is much bigger than the wavelength of the wave.

  • B: You get the maximum spreading or diffraction when the light waves pass through a gap of similar size to the wavelength of the incident waves.

  • You can see these effects with transverse water waves at the seaside as waves hit the protective walls of a harbour BUT you need a very tiny slit to observe diffraction with light waves because of their tiny wavelength.

  • When you hold up a fine needle towards a bright light, the edges aren't quite sharp because the light rays are diffracting around the pin's surface.

 


  • Check out your practical work you did or teacher demonstrations you observed in Unit P1.5, all of this is part of good revision for your module examination context questions and helps with 'how science works'.

    • reflecting light off a plane mirror at different angles,

    • carrying out refraction investigations using a glass block,

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