Question

based on the properties of light, explain how the image is formed on white cloth.​

Answer 1

Answer:

Visible light consists of electromagnetic waves that behave like other waves. Hence, many of the properties of light that are relevant to microscopy can be understood in terms of light’s behavior as a wave. An important property of light waves is the wavelength, or the distance between one peak of a wave and the next peak. The height of each peak (or depth of each trough) is called the amplitude. In contrast, the frequency of the wave is the rate of vibration of the wave, or the number of wavelengths within a specified time period (Figure 1).

Figure a shows a wavy line with evenly repeating waves upwards and downwards. A straight line through the center of the wavy line indicates the base of the waves. The distance from the peak of one wave to another is the wavelength. The distance from the baseline to the peak of a wave or the distance from the baseline to the trough of a wave is called the amplitude. Figure b shows three waves with unit time labeled across the bottom. The top line has waves that are widely spread apart. Waves with a wide wavelength have a low frequency. The bottom line has waves that are close together. Waves with a narrow wavelength have a high frequency. The middle line has a medium wavelength and therefore a medium frequency.

Figure 1. (a) The amplitude is the height of a wave, whereas the wavelength is the distance between one peak and the next. (b) These waves have different frequencies, or rates of vibration. The wave at the top has the lowest frequency, since it has the fewest peaks per unit time. The wave at the bottom has the highest frequency.

Interactions of Light

Light waves interact with materials by being reflected, absorbed, or transmitted. Reflection occurs when a wave bounces off of a material. For example, a red piece of cloth may reflect red light to our eyes while absorbing other colors of light. Absorbance occurs when a material captures the energy of a light wave. In the case of glow-in-the-dark plastics, the energy from light can be absorbed and then later re-emitted as another form of phosphorescence. Transmission occurs when a wave travels through a material, like light through glass (the process of transmission is called transmittance). When a material allows a large proportion of light to be transmitted, it may do so because it is thinner, or more transparent (having more transparency and less opacity). Figure 2 illustrates the difference between transparency and opacity.

Figure a shows the tips of a person’s gloved hands holding a clear plate with a lid. The plate contains a reddish material in the bottom of the plate. Figure b shows a piece of metal in a person’s hands. The material is dark with some shiny regions.

Figure 2. (a) A Petri dish is made of transparent plastic or glass, which allows transmission of a high proportion of light. This transparency allows us to see through the sides of the dish to view the contents. (b) This slice of an iron meteorite is opaque (i.e., it has opacity). Light is not transmitted through the material, making it impossible to see the part of the hand covered by the object. (credit a: modification of work by Umberto Salvagnin; credit b: modification of work by “Waifer X”/Flickr)

Light waves can also interact with each other by interference, creating complex patterns of motion. Dropping two pebbles into a puddle causes the waves on the puddle’s surface to interact, creating complex interference patterns. Light waves can interact in the same way.

In addition to interfering with each other, light waves can also interact with small objects or openings by bending or scattering. This is called diffraction. Diffraction is larger when the object is smaller relative to the wavelength of the light (the distance between two consecutive peaks of a light wave). Often, when waves diffract in different directions around an obstacle or opening, they will interfere with each other.

THINK ABOUT IT

If a light wave has a long wavelength, is it likely to have a low or high frequency?

If an object is transparent, does it reflect, absorb, or transmit light?

Lenses and Refraction

In the context of microscopy, refraction is perhaps the most important behavior exhibited by light waves. Refraction occurs when light waves change direction as they enter a new medium (Figure 3). Different transparent materials transmit light at different speeds; thus, light can change speed when passing from one material to another. This change in speed usually also causes a change in direction (refraction), with the degree of change dependent on the angle of the incoming light.

Picture a shows a light beam aimed at a piece of glass. When the light beam hits the transparent glass material it bends by approximately 45°. This bent light ray is the refracted ray. The opaque material which the glass is sitting upon does not have any light shining through it. Diagram b shows an arrow labeled incident ray pointing at a 45° angle down

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