1 E Flashcards
Cladding
layer of a lower refractive index material forming a protective coating around the inner core of a fibre optic cable
Dispersion
dispersion the separation of white light into its constituent colours due to the different refractive indices for different frequencies (colours) of light in a given medium
Fibre optic cable
fibre optic cable a single cable containing one or more optical fibres encased in cladding to protect it from the environment
Illusion
illusion a deceptive or misinterpreted sensory experience
Mirage
mirage an optical illusion caused by the refraction of light rays due to changes in air
temperature and pressure
Optic fibre
optical fibre a glass fibre that utilises total internal reflection to transmit light over long distances
Rainbow
rainbow an arch of colours caused by the dispersion of the Sun’s light through water in the atmosphere
White light
Not a particular colour or frequency of light, but a combination of the continuous visible spectrum of electromagnetic radiation. We perceive this combination of colours as white light and can observe the spectrum when we seperate teh light through a prism or lens
How does dispersion occur
The prism has
a different refractive index for different frequencies
of light.
Each time the
rays refract,
they separate further.
Light is dispersed into component colours i.e. a spectrum.
Roygbiv
What results in different angles of dispersion for the components of white light that transmits through a prism
Speed of light changes between mediums
Refractive index of a prism is dependent on the light frequency
How are rainbows formed
Rainbows are an application of dispersion and reflection where sunlight disperses through numerous raindrops, acting as prisms and reflecting this dispersed sunlight towards an observer’s eyes.
How are rainbows formed?
Rainbows are only visible when the following conditions are met:
• the Sun is behind the observer,
• there are water droplets in the air,
• the observer is far enough away from the water droplets,
• sufficiently bright sunlight reaches the water droplets to disperse and reflect light into the observer’s eyes.
Much like Figure 2, we can analyse the disp
Misconception
Total internal reflection always occurs within the raindrop.’
Most light that travels into a raindrop is lost back to the environment and only a small proportion of it reaches our eyes as a rainbow. This is because the internal reflection that is required for a rainbow to form is not total.
Rainbow perception
Much like Figure 2, we can analyse the dispersion of white light through a raindrop by looking at the extreme ends of the visible spectrum of light, red light and violet light (see Figure 4). The other colours fall between.
Each droplet in a rainbow disperses the whole spectrum of white light into a cone; however, the observer is so far away from the rainbow itself we do not see the whole rainbow from each raindrop. Every drop instead reflects one colour, dependent on its location, into our eye. The combination of potentially millions of raindrops each sending a single colour to our eyes is the rainbow (see Figure 5).
The combination of thousands of raindrops lead to a continuous spectrum of light being seen instead of seven blocks of colour.
In Figure 5, note that the raindrop reflecting red light is on top and violet on the bottom due to the different angles they refract from the raindrop. The different angles they disperse from the raindrops lead to the order we see the rainbow.
Why does different colours reach our eyes with rainbows in the same location
Due to our angle changing
How mirages form
Mirages occur only when the air has a range of temperatures between our eyes and the object we are looking at. Surfaces that easily heat up, such as roads or deserts, transfer that heat energy to the air above. This creates a gradient of both air density and temperature above the road (see Figure 6).
Air’s refractive index will be higher the more cold and dense it is. As light moves through air with this gradient of refractive indices its speed will change according to Snell’s Law. This causes the light to refract and change direction. When the light refracts enough towards an observer’s eyes, as shown in Figure 7, a mirage occurs.
The appearance of a reflection of cars or the sky or on the road is an optical illusion created because our brains assume all light travels in straight lines. In Figure 8,
the red ray represents the light reflected from the top of the tree.
Figure 7 A model representing how mirages occur
By extending a ray on the same angle as when it entered the eye, in the opposite direction, we see the tree reflected below where the actual object resides.3
Importantly, rays that travel through the temperature gradient change speed, and therefore bend, continuously with the change in temperature until either total internal reflection occurs or the light bends enough to turn into the observer’s eye.
