Light

Introduction

This topic explores the key concepts of light as they relate to:

• light as an entity
• vision
• rectilinear (straight-line) motion of light
• reflection of light
• image formation in plane mirrors
• refraction of light
• colour of light
• image formation in mirrors and lenses.

Key concepts of light

The activities in this topic are designed to explore the following key concepts:

• Light (radiant energy) is an entity that travels through space.
• Light travels very fast – 300 000 km/s in space.
• Light travels in straight lines.
• We see when reflected light is reflected from objects into our eyes.
• Some objects (e.g. a globe, the Sun, a flame)are sources of light; most things we see reflect light.
• Having two eyes is necessary for judgment of depth.
• Our brain puts together the stereo view we have of the world.
• Our eyes and brain can be misled.
• Light emitted from luminous objects keeps travelling until it is absorbed by something.
• Each point on a luminous object emits light in all directions (isotropic emission).
• Ordinary surfaces reflect/scatter light in all directions (diffuse reflection). Mirrors reflect light at an equal angle to the incoming light (specular reflection). Many surfaces, such as polished floors, both scatter and reflect light.
• Some surfaces reflect more light than others. Black surfaces reflect the least light.
• Light can be bent going into or out of water or glass; this causes images of different shapes and sizes.
• Light incident perpendicularly onto a transparent surface of a material does not change direction in passing through the material.
• Refraction of light is the change in speed as light passes from one transparent material into another.
• White light consists of all the rainbow colours (spectrum) (ROYGBV).
• The colour of an object is not a property of the object. It just reflects light of a particular colour and absorbs other colours.
• The primary colours of light are red, blue and green. The secondary colours of light are yellow, magenta and cyan.
• The image in a plane mirror is inverted, and symmetrical with the object.
• Our image in a plane mirror is equally far behind the mirror as we are in front of the mirror.
• Curved mirrors cause images of different sizes and shapes.
• Image formation in a lens or mirror requires that:
  • all the light that passes through a lens, or reflects off a mirror, contributes to the formation of an image
  • the light from each point on the object that passes through the lens, or is reflected from a mirror, either converges to the corresponding point on the image (real image), or appears to diverge from the corresponding point on the image (virtual image).

Students’ alternative conceptions of light and vision

Research into students’ ideas about this topic has identified the following non-scientific conceptions:

Light as an entity

• The effects of light are instantaneous. Light does not travel with a finite speed.
• Light is only associated with either the source or area of illumination. Rectilinear (straight-line) motion of light
• Light is associated only with either a source or its effects. Light is not considered to exist independently in space; hence, light is not conceived of as ‘travelling’.
• Light bends around objects, like clouds.
• Lines drawn outward from a light bulb represent the ‘glow’ surrounding the bulb.
• A shadow is something that exists on its own. Light pushes the shadow away from the object to the wall or the ground and is thought of as a ‘dark’ reflection of the object.
• Light actually consists of rays.

Vision

• The only condition to see an object is if light shines on the object. Light does not travel from the object to the eye.
• Something is emitted from the eye when looking at an object.
• Only the object and observer need to be bathed in light for the observer to see the object.
• The eye receives upright images.
• The lens is the only part of the eye responsible for focusing light.
• The lens forms an image (picture) on the retina. The brain then ‘looks’ at this image and that is how we see.
• The eye is the only organ for sight; the brain is only for thinking.

Isotropic emission of light from luminous objects

• Light is not necessarily conserved. It may disappear or be intensified.
• Light from a bulb only extends outward a certain distance, and then stops (or fades away). How far it extends depends on the brightness of the bulb.

Reflection of light

• Light reflects from a shiny surface in an arbitrary way.
• Light is reflected from smooth mirror surfaces but not from non-shiny surfaces.
• Curved mirrors make everything distorted.

Refraction of light

• Light always passes through a transparent material without changing direction.
• When an object is viewed through a transparent solid or liquid material, the object is seen exactly where it is located.

Colour of light

• A white light source, such as an incandescent or fluorescent bulb, produces light made up of one colour.
• Sunlight is different from other sources of light because it contains no colour.
• When white light passes through a prism, colour is added to the light.
• The rules for mixing colour paints and crayons are the same as the rules for mixing coloured lights.
• The primary colours for mixing coloured lights are red, blue and green.
• A coloured light striking an object produces a shadow behind it that is the same colour as the light. For example, when red light strikes an object, a red shadow is formed.
• The shades of grey in a black-and-white newspaper picture are produced by using inks with different shades of grey.
• When white light passes through a coloured filter, the filter adds colour to the light.
• The different colours appearing in coloured pictures in magazines and newspapers are produced by using different inks with all the corresponding colours.
• Colour is a property of an object, and is independent of both the illuminating light and the receiver (eye). For example, a red jumper contains red-coloured molecules.
• When a coloured light illuminates a coloured object, the colour of the light mixes with the colour of the object.

Image formation in mirrors and lenses

• A mirror reverses everything.
• For an observer to see the mirror image of an object, either the object must be directly in front of the mirror, or if not directly in front, then the object must be along the observer’s line of sight to the mirror. The position of the observer is not important in determining whether the mirror image can be seen.
• An observer can see more of his/her image by moving further back from the mirror.
• The way a mirror works is as follows: the image first goes from the object to the mirror surface. Then the observer either sees the image on the mirror surface or the image reflects off the mirror and goes into the observer’s eye.
• Students will often think about how a lens forms an image of a self-luminous object in the following way. They envision that a ‘potential image’ that carries information about the object leaves the self-luminous object and travels through space to the lens. When passing through the lens, the ‘potential image’ is turned upside down and may be changed in shape.
• Blocking part of a lens surface will block the corresponding part of the image.
• With self-luminous objects, the purpose of the screen is to capture the image so it can be seen. The screen is necessary for the image to be formed. Without a screen, there is no image.
• An image can be seen on the screen regardless of where the screen is placed relative to the lens. To see a larger image on the screen, the screen should be moved further back.
• An image is always formed at the focal point of the lens.
• The size of the image depends on the size (diameter) of the lens.