5.2 Light & EM Spectrum

The Electromagnetic (EM) spectrum is a continuous range of waves. They are all transverse waves that transfer energy.

Key properties of all EM waves:

• They all travel at the same speed in a vacuum (the "speed of light"): 300,000,000 m/s (or 3x10⁸ m/s).
• They do not need a medium to travel.

The spectrum is arranged in order of wavelength and frequency:

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• Radio waves (Longest wavelength, lowest frequency)
• Microwaves
• Infrared
• Visible light (Red, Orange, Yellow, Green, Blue, Indigo, Violet)
• Ultraviolet (UV)
• X-rays
• Gamma rays (Shortest wavelength, highest frequency, highest energy)

Source: Wikimedia Commons

• Radio waves: Used for communication (TV, radio broadcasting). They can travel long distances and bend (diffract) around hills.
• Microwaves: Used for cooking (they are absorbed by water molecules in food) and communication (satellites, Wi-Fi).
• Infrared: Used in thermal imaging cameras, heaters, and TV remotes.
• Visible light: Used for seeing and in photography.
• Ultraviolet (UV): Used in sunbeds, for sterilising equipment, and in security marking.
• X-rays: Used in medical imaging (to see bones) and airport security.
• Gamma rays: Used to sterilise medical equipment and in cancer treatment (radiotherapy).

Dangers of EM Waves

Higher frequency EM waves are more dangerous because they carry more energy and are a form of ionising radiation. This means they have enough energy to knock electrons off atoms, which can damage DNA and cause mutations or cancer.

• Microwaves: Can cause internal heating of body tissue.
• Infrared: Can cause skin burns.
• Ultraviolet (UV): Can cause skin cancer and eye damage.
• X-rays & Gamma rays: Are highly ionising and can cause cell mutations and cancer.

Reflection

Reflection is when a wave bounces off a surface.

The Law of Reflection states that the angle of incidence (i) = the angle of reflection (r). These angles are always measured from the "normal" line (an imaginary line at 90° to the surface).

Refraction

Refraction (transmission in the diagram) is the bending of a wave as it passes from one medium to another (e.g., from air to glass). This happens because the wave changes speed.

• When light enters a denser medium (like air to glass), it slows down and bends towards the normal.
• When light enters a less dense medium (like glass to air), it speeds up and bends away from the normal.

Source: Wikimedia Commons

A lens is a piece of transparent material that refracts light to form an image.

Converging (Convex) Lenses

A converging lens is thicker in the middle. It brings parallel rays of light together at a focal point (F). It can produce two types of images:

• Real Image: Formed where the light rays actually meet. A real image can be projected onto a screen. It is usually inverted (upside-down).
• Virtual Image: Formed where the light rays *appear* to come from. It cannot be projected. It is upright. (This is what a magnifying glass does).


Source: Wikimedia Commons

Diverging (Concave) Lenses

A diverging lens is thinner in the middle. It spreads parallel rays of light out, making them appear to come from a focal point. It always produces a virtual, upright, and smaller image.

Source: Wikimedia Commons

Converging Lenses

Drawing ray diagrams can get a bit tricky, but you can't go wrong if you follow these steps:

• Draw the converging lens, as seen in the image, with arrowheads pointing away from the lens. Draw an axis perpendicular to the lens, and mark the focal point.
• Draw the image.
• Draw a line parallel with the axis from the top of the image to the lens, and then from the top of the image to the point where the lens and axis meet (marked by 1) and 2) )
• Now from the parallel line 1), draw a line to the focal point and beyond the focal point 3).
• Repeat lines 1) 2) 3) from the bottom of the diagram
• Lines 2) and 3) meet on the other side of the lens, that is where your image will go. Where the black lines meet, this is the top of the image, and where the gold lines meet, this is the bottom.

How will my image turn out depending on how far from the focal length it is?

• If your image is further than 2 focal lenths, the produced image will be smaller and upside down
• If your image is at 2 focal lengths, the produced image will be the same size but upside down.
• Between 1 and 2 focal lengths, the image will be upside down and bigger
• Between the lens and the focal length, there will be a virtual image, bigger and the right way up, behind the real image (to the left on the diagram)

Diverging Lenses

• Draw the diverging lens as seen in the image, with arrowheads pointing inwards to the lens. Draw an axis perpendicular to the lens and mark the focal point.
• Draw the image
• Draw a line parallel with the axis from the top of the image to the lens, and then from the top of the image to the point where the lens and axis meet (marked by 1) and 2) )
• Now from the parallel line 1), draw a line away from the axis, marked by 3).
• Repeat lines 1) 2) 3) from the bottom of the diagram
• Now from both gold and black lines 3), draw a dashed line going backwards, marked by line 4).
• Where lines 4) and 2) meet marks the location of the image, gold being the bottom and black being the top. This makes a virtual image the right way up.

How will my image turn out depending on how far from the focal length it is?

For a diverging lens, the image will always be virtual, the right way up and smaller, no matter where from the focal point it stands.