When rays fail

The idea

When an opaque object in the path of light is very small, light tends to bend around it (diffraction); the straight-line ray treatment then fails, and light is treated as a wave.

A very thin hair

A small bulb and a card make a sharp, dark shadow. Now swap the card for something very, very small, like a single thin hair.

Will the hair still make a sharp, dark shadow?

• No: light bends around it and the dark shadow fades
• Yes: a sharp dark shadow, just thinner
• Yes: the hair stops even more light

Shrink the object

The top picture is the ray model: straight rays. The bottom picture is how light really behaves. Slide the object from big to very small.

Light bends around tiny things

If an opaque object on the path of light becomes very small, light has a tendency to bend around it and not walk in a straight line. This is the diffraction of light. The straight-line treatment of optics using rays fails, so light is then thought of as a wave.

In this chapter we use rays. They work well when the objects and mirrors are much bigger than light waves.

• Size of the object | What light does | Do rays work?
• Big | Goes straight: sharp, dark shadow | Yes
• Very small | Bends around it: no dark shadow (diffraction) | No: think of light as a wave

Notes

When an opaque object in the path of light is very small, light tends to bend around it (diffraction); the straight-line ray treatment then fails, and light is treated as a wave.

Check yourself

What does light do when it meets a very small opaque object?

When does the straight-line (ray) treatment of light fail?

A wardrobe and a single thin hair are placed, one at a time, in front of a small bulb. Which one makes a sharp, dark shadow?

How is the bending of light around a very small object explained?

• It goes straight and makes a sharp dark shadow. That happens for big objects. For a very small one the light bends round it.
• It bends around the object — correct. Yes! This is diffraction.
• It stops completely behind the object. Light bends around a very small object, so the space behind it is not completely dark.
• When the object is big and opaque. A big opaque object makes a sharp shadow: the ray treatment works well.
• Whenever light travels in air. Rays work well in air. Rays fail only around very small objects.
• When an object in the path of light is very small — correct. Yes! Light bends around it.
• The wardrobe — correct. Yes! It is big, so light cannot bend around it.
• The hair. A very small object lets light bend around it, so no sharp dark shadow.
• Both make the same sharp shadow. The hair is very small. Light bends around it, so the two shadows are not the same.
• By drawing more straight rays. More straight rays are still straight: they cannot bend. A new picture is needed.
• By thinking of light as a wave — correct. Yes! The details are studied in higher classes.
• By saying the object makes its own light. The object does not make light. It is the light that bends around it.