In this unit we will
- Identify the difference between the true appearance of objects and their appearance as viewed through optical instruments.
- Find examples of lenses in daily use and explain their functions.
- Design a tool using a lens.
Lesson 1: How Does Light Travel?
Facts:
- Light travels in waves in all directions but it travels in a straight line through the air.
- Light traveling in a straight line is called a ray.
- Light cannot bend to go around something.
- When light rays hit an object they are absorbed, reflected or refracted.
- We use the word reflect to describe when light bounces off an object.
- We use the word absorb to describe when light is soaked up by an object.
- We use the word refract to describe when light slows down as it passes through water.
- Transparent describes materials that allow light to pass through them easily.
- Translucent describes materials that only allow some light to pass through them.
- Opaque describes materials that do not allow any light to pass through them.
Lesson 2: What are Magnifying Glasses?
Facts:
- Magnifying glasses are used to make things look larger.
- Sometimes detectives’ use a magnifying glass to examine people’s finger prints.
- A magnifying glass is a convex lens.
- When light hits an object it bounces off the object.
- Light rays travel in straight parallel lines.
- We see things as light bounces off objects and travels to our eyes.
- When light passes through a magnifying glass the convex lens bends the parallel rays.
- After light rays pass through a magnifying glass they converge and create a virtual image on your eyes retinas.
- Magnifying glasses refract light rays (bend them) so they converge (come together).
- Magnifying glasses work by tricking our eyes into seeing something differently than it really is.
Lesson 3: Why do Objects Look Bent in Water?
Facts:
- Refraction of light is the change in direction of light when it passes from one medium to another obliquely.
- When light passes from water to air the light rays change direction i.e. they get refracted.
- When the light reaches our eyes, our eyes trace the light back as straight lines. Therefore, the rays of light seem to come from a point slightly above the original position.
- If we put a pencil into water this makes the pencil appear to be bent. It’s not really bent.
Lesson 4: What is Light Energy?
Facts:
- There are five types of energy available on Earth. These are sound energy, heat energy, wind energy, electrical energy and light energy.
- Light can be from a natural source or an artificial source.
- Natural sources of light include the Sun, the moon, stars, lightening and fire. (The moon actually reflects the light of the sun and does not produce its own light).
- Natural sources of light occur in nature naturally.
- Animals and plants that emit light are also natural sources of light and include fireflies, glow worms, jelly fish, flashy fungus, radiant railroad worms and glowing millipedes.
- Artificial light means it is made by humans beings.
- Artificial sources of light include L.E.D's, bulbs, fluorescent tubes, incandescent bulbs, halogen lamps, L.E.D lights, computers, mobile phones and toys.
- All living things need light.
- Plants use a process known as photosynthesis to convert sunlight into food which helps them grow.
- Cold-blooded animals need light to keep their body warm.
- Solar panels convert sunlight into electrical energy.
- Light travels in a wave but in a straight line.
- The Sun is one hundred and fifty million kilometres from Earth.
- Light from the sun takes eight minutes and seventeen seconds to reach Earth.
- Light enables us to see as it bounces off objects and travels to our eyes.
Lesson 5: What are Luminous and Non Luminous Objects?
Facts:
- We can see things around us because light is reflected from those objects.
- If there is no light in a room you will not be able to see anything as no light is falling on the objects in that room.
- During the daytime you can see things outside because light from the sun is shining on them.
- During the nighttime there is no sunlight so it is difficult to see things. However, light from the moon can help us to see things at night.
- Objects which produce light are known as luminous objects. The sun, stars and fire all glow so they are luminous objects.
- Living luminous objects include fireflies, jellyfish, flashy fungus, radiant railroad worms and glowing millipedes.
- We can see luminous objects as the light they emit enters our eyes.
- Objects which do not produce light are known as non-luminous objects. Glasses, doors and tables do not emit their own light so they are non-luminous objects.
- We can see non-luminous objects when they have light falling on them and the light is reflected into our eyes.
- Although the moon appears to emit light it is a non-luminous object as it is actually reflecting light that has fallen on to it from the sun.
- A non-luminous object with a smooth surface will reflect more light than a non-luminous object with a rough surface.
- Objects with a smooth surface appear shinier, brighter and glossier.
- Objects with a rough surface do not shine and appear dull.
Lesson 6: What are Transparent, Translucent and Opaque Objects?
Facts:
- When light hits an opaque object some of it is reflected and some of it is absorbed.
- If the object is opaque no light will pass through it and a shadow will be formed.
- Examples of opaque objects include footballs, batteries, pillows, teddy bears, plastic cups and pen stands.
- If the object is transparent light will pass through it.
- Examples of transparent objects include window panes, spectacles, car windshields, light bulbs, thermometers, aquariums, transparent sheets, ice and clean water.
- You can see objects on the other side of transparent objects because all the light passes through.
- If the object is translucent some light will pass through it and some will be absorbed or reflected.
- You can see through a translucent object but not completely.
- Examples of translucent objects include tinted windows, frosted glass, sunglasses, wax paper, a glass containing coloured liquid, oiled paper, coloured syrup and gemstones.
- A prism splits white Light into seven colours.
- The seven colours that make white light are red, orange, yellow, green, blue, Indigo and violet.
- When white light hits an opaque object some of the colours are absorbed and some are reflected.
- Often when white light hits an apple all the colours are absorbed except for the colour red. The colour red is reflected. So we see the apple as a red apple.
- Opaque objects which reflect all the light that hits them look white to us.
- Opaque objects which absorb all the light that hits them look black to us.
- If you wear black clothes on a sunny day you will feel very hot as black clothes absorb most of the light.
- If you wear white clothes on a sunny day you will feel cooler as white clothes reflect most of the light.
- You should wear light coloured clothes when it’s a hot day and dark coloured clothes when it’s a cold day.
Lesson 7: What is Specular and Diffused Reflection?
Facts:
- Reflection involves two rays, the incoming ray and the outgoing ray.
- The incoming ray is called the incident ray.
- The outgoing ray is called the reflected ray.
- The incoming and outgoing rays have the same angle on either side of the normal.
- The normal is an imaginary line at right angles to the surface of the object.
- When light falls on any surface some of it is absorbed and some of it is reflected.
- Reflected light can be specular or diffused depending upon the surface of the object.
- In specular reflection all the light is reflected in the same direction.
- If an object is smooth the reflected light is specular reflection and the image is clear.
- In diffused reflection the light is reflected in different directions.
- If an object has a rough surface the reflected light will be diffused reflection and the image will be disrupted.
- You can only see your image in objects with smooth surfaces as all the light is reflected from them in the same direction.
Lesson 8: What is Refraction of light?
Facts:
- In a single medium when light reflects the angle of incidence equals the angle of reflection.
- When light travels from one medium to another the light changes speed and it changes direction.
- Refraction of light is sometimes called bending of light.
- Refraction of light occurs when light moves from one medium to another medium.
- The refraction can happen as light moves from air to water or water to air.
- Refraction can also happen when light moves from air to glass or glass to air.
- When light enters into a prism its speed slows down and it changes direction.
- When light leaves a prism its speed increases and it changes direction.
- When light enters a denser medium it slows down and bends towards the normal.
- When light leaves a dense medium it speeds up and bends away from the normal.
- When we see a pencil bent in water we are really seeing a virtual image of the pencil.
- Birds look as if they are split into two halves in water because the bottom half of the bird is a virtual image.