Thursday, 2 August 2012

What Happens When Light Falls On a Surface

What happens when light reaches a surface depends on various factors.
  •     The angle at which light is falling on the surface
  •     Color temperature of the light
  •     Texture, tone and color of the surface

Different materials affect light differently

Opaque Materials

When light falls on an opaque material; depending upon the color and texture of the surface of the material, some light is reflected back and the rest is absorbed in the form of heat. The darker the colors tone of the surface, greater the amount of light that is absorbed and vice versa. So darker surfaces reflect less light and lighter surfaces reflect more light.

The color of the reflected light will depend on the color of the surface material. For example when white light falls on a blue board what happens is the blue surface reflects the blues from the spectrum and absorbs all the rest. That is the reason why it appears blue. Similarly a red surface reflects red and a green surface reflects green.

Now what happens if the light falling on the surface is not white?

For example what if the light falling on our blue board is red and not white, so it has no blue spectrum to reflect and it absorbs all the rest – result is that our blue board appears black when viewed in red light. A basic understanding of how different color light reacts in different situations will be of much help when using gels or color filters or a combination of both to generate various effects.
Colour of Surface on Which Light Falls
Colour of Surface on Which Light Falls

The texture of the surface on which the light is falling is also an important element determining how light behaves after reaching the surface. A surface with a matt finish such as a drawing paper or an egg shell etc scatters light evenly so that the angle from which the light is reaching the surface is of less importance.

Reflection off a Matt Surface (Diffuse Reflection)
Reflection off a Matt Surface (Diffuse Reflection)
But in contrast if the surface is shiny or smooth such as glass or glossy paint, the surface acts more like a mirror reflecting most of the light back in one direction (the same angle at which the light reached the surface) resulting in a phenomenon named Specular Reflection.

Reflection off a Shiny Surface (Specular Reflection)
Reflection off a Shiny Surface (Specular Reflection)

Most of you must have noticed the patch of glare when picture of a glass window or a glossy subject is taken with the on camera flash on. That is because when light strikes a shiny surface at right angles, it is reflected backward along its original path. The trick to avoid such patches is to realign your camera in an angle to the subject so that the reflected light coming off the subject is angled in any other direction rather than straight back at the camera.

Transparent or Translucent Materials

Materials that transmit light directly through them are called transparent materials and those materials that allow light to pass through them but in the process diffuse the light are termed translucent materials. Clear glass, water etc are examples of transparent materials while tracing paper, ground glass etc are examples of translucent materials. In the case of both these materials if the material is colored then it will allow more of those wavelengths to pass through. Thus a red transparent or translucent material will allow more of red to pass through. And if the light reaching the red material is green or blue, then the material will almost have the effect of an opaque object.

Selective Transmission of Light
Selective Transmission of Light

However there is one difference in the character of light that passes from transparent and translucent materials. Translucent materials scatter the light and appear evenly illuminated than clearer materials. Also the light coming out from translucent materials will be more diffused.
Direct Transmission of Light
Direct Transmission of Light

Diffused Transmission of Light
Diffused Transmission of Light


The change of light path when light travels obliquely from one transparent medium to another is known as refraction. To see refraction at work simply dip a pencil into a bowl of water, the pencil will appear bent at the water surface.

Refraction - Photo By  Bill Gracey

What causes Refraction?

The density of the medium through which light travels affects its speed. As a result when light passes obliquely from air into some other transparent material, as glass or water, its wavefront becomes slowed unevenly. And new straight line path is established.

However one thing to remember is that refraction only bends oblique light. Light that falls on the edges of two transparent materials at right angles slows minutely but does not change direction. And most light falling on the edge at a very low angle (very oblique) is reflected back off the surface.
The Whole Picture

Everything we see in the world around us appears the way it does because of light and its properties, diffusion, absorption, reflection, refraction, highlights, textures, colors, etc. From years of experience our eyes and brain recognize all these subtle light signals to signify solidity, roundness, textures etc and we get to know things the way we have always known them.

In the next article we will discuss about Quality of Light.

Related Reading

  1. An Introduction to Light
  2. Wavelengths and Colours
  3. Quality Of Light
  4. Light And Colour
  5. White Light And Primary Colours