Allen HJ Yang

When light hits an object, one of three things can happen to it.

Light can be reflected, where it bounces off at the same angle that you hit it at. A mirror is an object that bounces off most of the light that hits it and is the source of its reflective nature.

A quick experiment that you can do is to take a flashlight and point it directly at a mirror. If you did this right, you probably got blinded a bit. The light is bouncing straight back into your eyes. Now, move a bit to the left, but point your flashlight straight out from your body towards the point you were aiming at before. The light from your flashlight will appear to light up a place that is not the mirror you are pointing it at. As you keep moving to the left while aiming at the same point on the mirror, the spot reflecting back from the mirror should be moving away from you. That is because mirror is reflecting the light at the SAME angle you are hitting it at. So, the wider the angle that light hits a mirror, the farther away from its starting point it will end up.

Light can also be absorbed, where the energy of the light is largely transferred into heat. In this case, think of a black asphalt surface on a sunny day. The majority of the light is being absorbed by the asphalt and being turned into heat. This is why it appears black, because very little light is reflecting off the surface into your eyes, and the absorbed heat is why asphalt can burn you on a hot, sunny day.

The last thing light can do when it hits an object is to change the direction it is moving in, more like bending light than bouncing light (as in reflection). This phenomena is known as refraction. When light travels through anything other a perfect vacuum, the speed of light is decreased by some amount. Different materials slow light down by different amounts. At the interface between two materials, light compensates for the differences in speeds by changing the direction it is moving in. This refraction occurs at every interface between materials, be it air-water, air-glass, water-plastic.

How much light bends at the boundary is measured by something known as the refractive index, which measures how much the speed of light changes in a material compared to vacuum. By proxy, then it tells us also how much light will bend at the interface between the two materials.

For interested readers who want to know more about how this actually works, I refer you to the refraction page on Wikipedia, which does a pretty good job of explaining refraction in more detail.

The optical properties of any material can be summed up by how it reflects, absorbs, and refracts light. A material is generally considered transparent if it has a low amount of reflection and absorption. A mirror is an object that has a high level of reflection, but little absorption or refraction. An opaque material (something you can’t see through or see your own reflection in) is a material that has a high degree of absorption.

This entry was posted on Wednesday, February 27th, 2008 at 10:33 pm and is filed under SciWiki. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.