Reflection and Refraction

Diffraction is the spreading out of waves when they pass through a gap. The extent to which this happens depends on the wavelength and size of the gap.

The law of reflection

Sound waves and light waves reflect from surfaces. The angle of incidence equals the angle of reflection. This is called the law of reflection. So, if a wave hits a mirror at an angle of 36°, it will be reflected at the same angle (36°).

You can investigate the law of reflection using a light box, mirror and angle measurer.

Wave striking a water barrier and a ray of light striking a plane mirror

An incident ray of light hits a plane mirror at an angle and is reflected back off it. The angle of reflection is equal to the angle of incidence. Both angles are measured from the normal. The normal is an imaginary line at right angles to the plane mirror.

Smooth surfaces produce strong echoes then sound waves hit them, and they can act as mirrors when light waves hit them. The waves are reflected uniformly and light can form images.

The waves can:

• be focused to a point, eg sunlight reflected off a concave telescope mirror
• appear to come from a point behind the mirror, eg a looking glass

Rough surfaces scatter sound and light in all directions. However, each tiny bit of the surface still follows the rule that the angle of incidence equals the angle of reflection.

Refraction

Sound waves and light waves change speed when they pass across the boundary between two substances with different densities, such as air and glass. This causes them to change direction and this effect is called refraction. We can use water waves in a ripple tank to show this effect.

Refraction doesn't happen if the waves cross the boundary at an angle of 90° (called the normal) - in this case, they carry straight on.

The refraction follows a regular pattern.

When light passes from air into semi-circular blocks of Perspex the following is seen:

The light enters the curved face of the block directly, so no refraction is seen here. As you increase the angle of incidence you see a greater angle of refraction.

At a specific angle, the light ray will no longer leave the block. At this point the angle of incidence is called the critical angle. Any further increase in the angle of incidence will mean the ray is reflected, not refracted.

When white light passes from air into a triangular prism, it is refracted as it enters, and then again as it exits. As it leaves the prism, the different wavelengths of the individual colours of light result in different angles of refraction.

This splits white light into the seven colours of the rainbow. This process is called dispersion. Red light is refracted the least and violet the most. Each colour of light can be called monochromatic.

Refraction from a prism

Efficiency of Energy Transfer

"Wasted" energy

Energy cannot be created or destroyed. It can only be transferred from one form to another, or moved. Energy that is "wasted", like the heat energy from an electric lamp, does not disappear. Instead, it is transferred to its surroundings and spreads out so much that it becomes difficult to do anything useful with it.

Electric lamps

Ordinary electric lamps contain a thin metal filament that glows when electricity passes through it. However, most of the electrical energy is transferred as heat rather than light energy. This is the Sankey diagram for a typical filament lamp.

Sankey diagram for a filament lamp

Modern energy-saving lamps work in a different way. They transfer a greater proportion of electrical energy as light energy. This is the Sankey diagram for a typical energy-saving lamp.

Sankey diagram for a typical energy-saving lamp

From the diagram, you can see that much less electrical energy is transferred, or "wasted", as heat energy.

Calculating efficiency

The efficiency of a device such as a lamp can be calculated using this equation:

efficiency = (useful energy transferred ÷ energy supplied) × 100

-The efficiency of the filament lamp is 10 ÷ 100 × 100 = 10%.

This means that 10% of the electrical energy supplied is transferred as light energy. 90% is transferred as heat energy.


-The efficiency of the energy-saving lamp is 75 ÷ 100 × 100 = 75%.

 This means that 75% of the electrical energy supplied is transferred as light energy. 25% is transferred as heat energy.

(Note that the efficiency of a device will always be less than 100%.)

The Electro-Magnetic Spectirum

You should remember from your Key Stage 3 studies that white light can be split up into a spectrum of many different colours. You should know that visible light is just part of a continuous spectrum of electromagnetic radiation. Different types of electromagnetic radiation have different hazards and uses.

What is a spectrum?

The visible spectrum

Refraction from a prism

Using a prism, you can split up white light to form a spectrum. A prism is a block of glass with a triangular cross-section. The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

Visible light is just one type of electromagnetic radiation. There are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through space.

THE ELECTROMAGNETIC SPECTRUM

X-rays, visible light and radio waves are all types of electromagnetic radiation.

The main types of electromagnetic radiation

there are...
-RADIO WAVES
-MICRO WAVES
-INFARED
-VISABLE LIGHT
-ULTRA VIOLET

-X RAYS
 -GAMMA RAYS

All types of electromagnetic radiation:

• are transverse waves
• travel at the same speed in a vacuum - empty space

The speed of electromagnetic radiation in a vacuum is 299,792,458 m/s. This is approximately three hundred million metres per second - nearly nine hundred thousand times faster than sound, which is why you see a flash of lightning before you hear the thunder.

Hazards of electromagnetic radiation

Over-exposure to certain types of electromagnetic radiation can be harmful. The higher the frequency of the radiation, the more damage it is likely to cause to the body:

• microwaves cause internal heating of body tissues
• infrared radiation is felt as heat and causes skin burns
• X-rays damage cells, causing mutations (which may lead to cancer) and cell death
• gamma rays also damage cells, causing mutations (which may lead to cancer) and cell death

Microwaves

Microwave radiation can be used to transmit signals such as those for mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile phones in their range.

Ultraviolet light

Ultraviolet radiation - UV - is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens as our bodies attempt to reduce the amount of ultraviolet radiation reaching deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important, because ultraviolet radiation can cause normal cells to become cancerous.

Uses of electromagnetic radiation

Scanning by reflection

The coloured part of your eye, the iris, has a unique pattern. Visible light reflected off the iris can be analysed by a computer and compared to stored patterns. In this way, people can be identified by their iris pattern.

Scanning by emission

All objects give off or emit infrared radiation. The hotter the object, the more infrared radiation it emits. Sensors that can detect this radiation allow the temperature of objects to be measured. This is useful for medical scans to detect abnormally hot parts of the body, which may indicate damage or illness. The detection of infrared emissions is also useful for tracking criminals at night and for military 'night vision' goggles.

Scanning by absorption

Materials absorb different types of electromagnetic radiation to different degrees.

For example X-rays are weakly absorbed by skin and muscle, but strongly absorbed by dense material such as bone and metal. This means that X-rays can be used to produce images of bones inside your body, to check for damage such as fractures.
X-rays are also used in industry to check metal components and welds for cracks or other damage.
Microwave radiation is absorbed by water molecules, so it can be used for cooking. Water in the food absorbs the microwave radiation, which causes the water to heat up and cook the food.
Microwaves are also used to monitor rain. Satellites have been set up to measure how much microwave radiation, sent from ground-based transmitters, is absorbed by water in the atmosphere. Computers analyse the information and build up a picture of the rain and cloud.
Ultraviolet light is used to detect forged bank notes. Certain chemicals in paper absorb ultraviolet light and emit the energy as blue light. Real bank notes have invisible markings that only show up using ultraviolet light.

Home Insulation

There are a few different ways of insulating your home.

LOFT INSULATION
It is fireglass 'wool' laid across the loft floor which reduces conduction through the cieling into the roof space.

HOT WATER TANK JACKET
Reduces conduction through the water tank.

CAVITY WALLS
Two layers of bricks which foam between them minimise the convection.

DOUBLE GLAZING
Two layers of glass with air between them.

DRAUGHT PROOFING
Strips of foam and plastic around doors and windows stop hot air from getting out.

THICK CONTANS  
Reduce conduction and radiation through the windows.

The Song

https://www.youtube.com/watch?v=bjOGNVH3D4Y

Do you want to know about the electromagnetic spectrum, then listen to this song. trust me it will help you!

Ultra Violet Radiation

Ultraviolet radiation is found naturally in sunlight. Exposure to ultraviolet radiation can cause our skin to tan. It can also cause:

• Sunburn

• Skin cancer

• Eye cataracts

• Premature ageing of the skin.

There is a public health issue about ultraviolet radiation from the Sun and sun beds. Health education programs aim to inform people about the dangers of ultraviolet radiation. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet radiation that reaches deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important because ultraviolet radiation can cause normal cells to become cancerous.

Sunscreens can reduce the damage caused by ultraviolet radiation. They contain chemicals that absorb a lot of the radiation and prevent it from reaching our skin. They may also contain chemicals that reflect some of the radiation away from the skin. Manufacturers of sunscreens make products with different sun protection factors, SPFs: the higher the factor, the longer you can stay out in the sun without burning; high factor sunscreens reduce the risks from ultraviolet radiation more than low factor sunscreens.

If, for example, you would get sunburnt after ten minutes in the sun, with Factor 5 applied you could stay in the sun for 50 minutes - or for 500 minutes with Factor 50 applied. But the real time is usually lower, because some of the sun block gets absorbed by the skin, and some gets rubbed off.

The ozone layer is the part of the upper atmosphere where ozone is found in the highest concentrations. The ozone there absorbs ultraviolet radiation, preventing most of it from reaching the ground. This is important because ultraviolet radiation can lead to skin cancer.

Near the end of the last century, scientists discovered that ozone levels over the Antarctic were reduced. This discovery was unexpected. Chemists knew that reactive chlorine atoms could destroy ozone. They also knew that chemicals called chlorofluorocarbons - CFCs - break down in ultraviolet light to release reactive chlorine atoms. Scientists used these ideas to explain the low ozone levels.

CFCs were once used widely in insulating foam and aerosol spray-cans. Once released, they gradually spread through the atmosphere, eventually reaching the ozone layer. Once there, they destroy ozone. CFCs have now been almost completely replaced by chemicals that do not cause this damage.