Fun-damentals of Light: For Non-scientists Quiz

Answer: Cosmic rays

The electromagnetic spectrum starts with long wavelength radio waves and continues as the wavelengths get shorter to microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays. Cosmic rays are high energy particles, especially protons and atomic nuclei, which move through space.

Answer: Both waves and particles

Light and all electromagnetic spectrum (EMS) radiation primarily has wave properties. A changing electric field creates a changing magnetic field and vice versa. Because of this, these fields can propagate through space in the form of electromagnetic waves. However, it was shown that light does not come in infinite continuous amounts as it would if it was purely a wave.

Einstein postulated that the energy of EMS radiation comes in discrete amounts, or quanta, rather than continuous amounts. These quanta of EMS energy are called photons. Because of the quantization of energy, under certain conditions, light behaves as a particle. Therefore, light and EMS radiation in general, have both wave and particle properties.

Answer: False

The speed of light in a vacuum is constant. However, light goes at slower speeds when it is traveling in denser materials. This is analogous to when we walk in water versus walking through air as we usually do. The speed of light in water is about 3/4 its speed in a vacuum and its speed in glass is about 2/3 its speed in a vacuum.

The slowing of light in denser media is mainly due to the interaction between light and the electrons in the material. The more electrons the light interacts with, the more slowing occurs.

Answer: False

If light is traveling in the same material, it travels in a straight line. However, when it transitions into a different material (with a different index of refraction) at an angle, the light bends. This is called refraction. It is because of refraction that optics are possible. When light traveling through the air enters glass, it slows and bends to a greater angle than the angle at which it enters. The mathematics of the amount of bending is governed by Snell's law.

By forming lenses of varying shapes, it is possible to bend light such that images are magnified (or reduced in size but with a wider field of vision) and focal points are altered. This makes telescopes, microscopes and eyeglasses possible. Refraction also produces some interesting natural optical phenomena including rainbows and mirages.

Answer: True

Henry Cavendish noted in 1784 that Newton's laws of gravitation predicted that light would bend around massive objects like the sun. However, Einstein realized that calculations based on Newtonian principles were incomplete. He postulated that his general theory of relativity accurately calculated the effect of massive objects on the direction of light.

During the total solar eclipse in 1919, stars near the Sun were being observed. The observations demonstrated that the light from stars passing close to the Sun was slightly bent, so that stars appeared slightly out of position. The observations confirmed the predictions of Einstein's theory. The result was fantastic news and was headlined on the front page of most major newspapers making Einstein and his theory of general relativity world famous.

In general relativity, light follows the curvature of space and time (the 3 dimensions of space plus the dimension of time). Massive objects bend spacetime so that when light passes by a massive object, its path is curved toward the object. This means we can observe that the light from a source on the other side of a massive object will be bent towards an observer's eye, just like an ordinary glass lens would do. This effect is therefore known as gravitational lensing.

Because of general relativity, it is more accurate to say that light always travels along the shortest path in spacetime. When massive objects bend spacetime, light takes the shortest spacetime path and travels along the curvature.

Answer: Yellow light

The spectrum of light (a.k.a. the colors of the rainbow) starts with the longest wavelength, red, and progresses to orange, yellow, green, blue, indigo and violet. Adding two equal intensity colors together produces a color halfway in between the two in the Munsell color system which is based on human perception of color. Red plus green makes yellow. Green plus blue make cyan (light blue). Red plus blue makes magenta.

Red, green and blue are known as primary colors because adding all three produces white light. Yellow, cyan and magenta are known as secondary colors since they are produced by adding two primary colors. Adding light gives a very different effect compared to adding pigments. Adding different colors of pigments produces darker colors, that is, more towards brown and eventually black.

Answer: White

The Sun radiates white light, the full spectrum of colors. The reason it appears yellow on earth is due to the same reason that causes the sky to appear blue. Gas molecules in the atmosphere scatter shorter wavelength blue light more so than longer wavelength red light. This phenomenon is called Rayleigh scattering. The Sun then appears yellow because the scattering has caused the blue light to be removed from the appearance of the Sun.

Adding the three primary colors of red, green and blue produces white light. Subtracting the blue leaves red and green which, when combined, make yellow on the Munsell color chart for light.

Answer: False

The speed of light in materials denser than a vacuum is slower. Yet it is possible for particles, such as electrons, to move faster than the light does in these materials. Matter can be accelerated to a velocity higher than that of light (although still less than the speed of light in a vacuum) during nuclear reactions and in particle accelerators. The resulting effect is called Cherenkov radiation.

In situations such as nuclear reactors which are under water, a characteristic glowing blue light is emitted because of the particles traveling faster than the speed of light in water. The effect is named for Pavel Cherenkov who first observed it in 1934 and was one of the winners of the Nobel Prize in 1958 for his efforts.

Answer: True

Most substances start to glow red at around 525 °C (977 °F). As the temperature rises, the glow becomes brighter and its color changes from red towards white and finally blue.

The classic example of incandescence is the incandescent light bulb. Electrical current passes through a filament. Because of the filament's resistance, it heats up and glows.

Everyday flames with relatively low temperatures, such as those produced by candles, also emit light by incandescence. These flame produces light because of tiny bits of incompletely burned material, called soot, which is heated up and glows.

Answer: True

Luminescence generally involves the excitation of atoms or molecules into higher energy states. These higher energy states are unstable, so the atoms or molecules drop back into lower energy states by emitting photons. Because these energy states are in discrete amounts, like a ladder, the photons emitted are always at a specific wavelength based on the distance between the energy states. If this wavelength is in the visible spectrum of light, a specific color is generated.

The classic example of luminescence is the neon light. To create neon light, a glass tube with metal ends is filled with neon gas at low pressure. When high voltage is applied to the tube, the result is that atoms of neon are energized into higher energy states. When the atoms drop back down into their normal energy state, they emit a characteristic orange light.

A wide variety of forms of energy, including chemical reactions, can produce light by non-thermal processes . For example, it is a chemical reaction that creates light in instances of bioluminescence such as with lightning bugs. Different animal species have different chemicals and may therefore produce different colors of light.