Uses of Visible Light and Ultraviolet

To see an object, visible light must first reflect off its surface and enter your eye through an opening called the pupil. Next, a converging (convex) lens refracts this incoming light to focus it precisely onto the retina at the back of the eye. Photosensitive cells in the retina then convert this absorbed light energy into electrical signals. Finally, these electrical signals travel along the optic nerve to the brain, which processes the image.

The Mechanism of Photography

Cameras capture images by closely mimicking the anatomy of the human eye. First, a converging (convex) lens focuses incoming light to create a real, inverted image. In modern digital cameras, this image lands on a CCD (Charge-Coupled Device) or CMOS sensor instead of a retina. These electronic sensors contain millions of tiny pixels (photosites) that transform light energy directly into electrical signals.

Alternatively, traditional analogue cameras focus light onto photographic film containing silver compounds, which undergo a permanent chemical change when struck by light. In both digital and film photography, the amount of light entering the camera is controlled by the aperture (the size of the opening), while the duration of exposure is controlled by the shutter speed.

Comparison: The Human Eye vs. The Camera

Feature	Human Eye	Digital Camera / Film
Focusing light	Converging (convex) lens	Converging (convex) lens
Controlling light entry	Pupil	Aperture
Detecting/Recording image	Retina	CCD / CMOS sensor or photographic film

Illumination and Communication

Artificial sources like LEDs and filament bulbs provide illumination by emitting visible light that reflects off surrounding objects into our eyes. In commercial agriculture, highly intense artificial visible light is used to illuminate greenhouses continuously, maximising photosynthesis to increase crop yields. Furthermore, visible light pulses are used to transmit data through optical fibres at incredibly high speeds using a phenomenon called total internal reflection.

Visible Light Calculations

Because visible light is an electromagnetic wave, its properties can be calculated using the standard wave equation:

$$v = f \times \lambda$$

Where:

• $v$ = wave speed ($\text{m/s}$)
• $f$ = frequency ($\text{Hz}$)
• $\lambda$ = wavelength ($\text{m}$)

Worked Example

Calculate the frequency of a green light wave with a wavelength of $500 \text{ nm}$ ($5 \times 10^{-7} \text{ m}$) travelling in a vacuum ($v = 3 \times 10^8 \text{ m/s}$).

Step 1: Rearrange the wave equation to make frequency the subject.

• $f = \frac{v}{\lambda}$

Step 2: Substitute the known values.

• $f = \frac{3 \times 10^8}{5 \times 10^{-7}}$

Step 3: Calculate the final answer with units.

• $f = 6 \times 10^{14} \text{ Hz}$

Understanding Ultraviolet (UV) Radiation

You cannot see the hidden security markings on a passport under normal room lighting, but they appear glowing bright under the right conditions. This is because these markings react to ultraviolet radiation, which sits just above visible light on the electromagnetic spectrum. UV has a higher frequency and a shorter wavelength than visible light, meaning it carries significantly more energy.

The Mechanism of Fluorescence

Ultraviolet radiation interacts with certain materials through a process called fluorescence. First, an atom absorbs the high-energy UV radiation, causing its electrons to undergo excitation and jump to a higher energy level. As these excited electrons rapidly drop back down to their original, more stable energy states, they release the stored energy as visible light. This emitted light has a longer wavelength than the absorbed UV radiation.

Uses of Ultraviolet: Fluorescent Lamps

Fluorescent lamps are significantly more efficient than traditional filament bulbs, converting around 50% of electrical input into useful light compared to less than 10%. First, an electric current flows through a glass tube containing mercury vapour, exciting the mercury atoms so they emit UV radiation. Next, this invisible UV radiation strikes a phosphor coating on the inside surface of the glass tube. Finally, the phosphor coating absorbs the UV rays and fluoresces, emitting bright, visible light into the room.

Uses of Ultraviolet: Security and Forgeries

Official documents like banknotes, passports, and ID cards are printed using fluorescent inks that remain completely invisible under normal visible light. However, when illuminated by a UV lamp, these special inks absorb the UV radiation and fluoresce, emitting visible light to reveal hidden numbers or intricate patterns. Similarly, police advise homeowners to use UV pens to write postcodes on valuables; the ink cannot be seen by thieves but is instantly visible under a police UV scanner.

Uses of Ultraviolet: Disinfecting Water

Ultraviolet radiation is a powerful tool for sterilisation. Water treatment plants use high-energy UV-C radiation to kill harmful bacteria and viruses without needing chemical additives like chlorine. The UV rays are absorbed directly by the DNA or RNA of the microorganisms, violently breaking their vital chemical bonds.

This genetic damage permanently inactivates the pathogens, meaning they can no longer replicate or cause disease. For this mechanism to work effectively, the water must have low turbidity (it must be very clear) so that floating particles do not shade the bacteria, and the water must flow slowly enough to receive a sufficient dosage of UV radiation.

Worked Example

A water treatment system uses a UV lamp with an intensity of $15 \text{ mW/cm}^2$. If the water is exposed for $5 \text{ s}$, calculate the UV dosage received. (Formula: Dosage = Intensity \times Time)

Step 1: Identify the values.

• $\text{Intensity} = 15 \text{ mW/cm}^2$, $\text{Time} = 5 \text{ s}$

Step 2: Substitute into the equation.

• $\text{Dosage} = 15 \times 5$

Step 3: Calculate the final answer with units.

• $\text{Dosage} = 75 \text{ mJ/cm}^2$