Optics Flashcards
All 30 cards in this deck
What is the law of reflection?
Angle of incidence = angle of reflection (both measured from the normal).
True or false? The angle of incidence is the angle between the ray and the surface of the glass block.
False. It is the angle between the incident ray and the normal (the line at 90° to the surface).
What happens to the speed and direction of a light ray as it passes from air into glass?
e.g. a ray hitting a glass block at 40° to the normal
It slows down and bends towards the normal, so the angle of refraction is less than 40°.
Why does white light split into a spectrum of colours when it is refracted by glass?
The different wavelengths travel at different speeds in the glass, so they refract (bend) by different amounts.
What is total internal reflection?
When light inside a denser medium hits the boundary at an angle of incidence greater than the critical angle, all of it is reflected back into the medium and none is refracted out.
What is the critical angle?
The angle of incidence (inside the denser medium) for which the angle of refraction is 90°.
In a ray diagram, what happens to a ray in glass that hits the glass–air boundary at exactly the critical angle?
The refracted ray travels along the boundary (angle of refraction = 90°).
True or false? Total internal reflection can happen when light travels from air into glass.
False. It only happens going from the slower, denser medium into the faster, less dense one, e.g. glass into air.
True or false? When the angle of incidence inside glass is less than the critical angle, light is refracted out of the glass at the boundary.
True. Below the critical angle most of the light refracts out, bending away from the normal.
What is specular reflection?
Reflection from a smooth surface, where parallel rays are all reflected in the same direction, giving a clear image.
What is diffuse reflection?
Reflection from a rough surface, where rays are scattered in many different directions, so no clear image is formed.
True or false? Diffuse reflection happens because the law of reflection is not obeyed at a rough surface.
False. The law of reflection is obeyed at every point, but the surface normals point in different directions, so the rays scatter.
What decides the colour of an opaque object seen in white light?
The wavelengths its surface reflects; the other wavelengths are absorbed.
e.g. a red object reflects red light and absorbs the rest.
What does a colour filter do to white light passing through it?
It transmits its own colour(s) of light and absorbs the other wavelengths.
e.g. a blue filter transmits blue and absorbs the rest.
True or false? A white object looks white because it absorbs all the wavelengths of visible light.
False. It reflects all wavelengths of visible light roughly equally; a black object absorbs them.
What is the equation linking the power of a lens and its focal length?
power (in dioptres)
What is the focal length of a lens?
The distance from the centre of the lens to its principal focus.
How is the shape of a lens related to its power?
A fatter (more strongly curved) lens has a shorter focal length and so a greater power.
True or false? A lens with a longer focal length has a greater power.
False. Power is focal length, so a longer focal length means a lower power.
What shape is a converging lens, and what does it do to rays parallel to the principal axis?
Convex — thicker in the middle; it refracts the parallel rays so they meet at the principal focus.
What shape is a diverging lens, and what does it do to rays parallel to the principal axis?
Concave — thinner in the middle; it spreads the rays out so they appear to come from the principal focus.
What is a real image?
An image formed where light rays actually meet, so it can be captured on a screen.
What is a virtual image?
An image that light rays only appear to come from when traced back, so it cannot be formed on a screen.
What is the nature of the image formed by a converging lens when the object is further from the lens than the focal length?
Real and inverted, formed on the other side of the lens.
What is the nature of the image formed by a converging lens used as a magnifying glass (object closer than the focal length)?
Virtual, upright and magnified, on the same side of the lens as the object.
What is the nature of the image formed by a diverging lens?
Always virtual, upright and diminished, on the same side of the lens as the object.
What are the steps to draw a ray diagram locating the image formed by a converging lens?
e.g. an object standing on the axis beyond the focal point
- Draw a ray from the top of the object parallel to the axis; it refracts through the principal focus on the far side.
- Draw a ray from the top of the object straight through the centre of the lens.
- The image top is where the two rays cross — here a real, inverted image.
What are the steps to trace a light ray through a rectangular glass block in the core practical?
e.g. using an angle of incidence of
- Draw round the block and draw the normal at the entry point.
- Shine a ray at to the normal; mark the ray going in and coming out with dots.
- Remove the block, join the dots and measure the angle of refraction from the normal.
In the refraction core practical, why is the light source used with a narrow slit?
It produces a single thin ray, so its path can be marked and measured accurately.
In the refraction core practical, what is plotted on each axis of the results graph?
Angle of refraction on the -axis against angle of incidence on the -axis.