Electromagnetic Waves and Light: Properties and Phenomena
Electromagnetic Waves and Light
Electromagnetic waves propagate at the same speed as light in a vacuum. Therefore, light is an electromagnetic wave that propagates without material support.
Examples of electromagnetic waves:
- Radio waves
- Microwaves
- Infrared radiation
- Visible light (500nm)
- Ultraviolet radiation
- X-Rays
- Gamma rays
Corpuscular Theory
According to Newton, light is formed by separate particles of different colors and sizes emitted by luminous bodies that reach our eyes.
This theory is grounded in the concept of light traveling in a straight line. This is because the corpuscles spread rapidly in a straight line.
Shadows
When moving corpuscles do not cross a barrier, they produce a shadow.
Reflection
When corpuscles clash with a polished surface, they bounce, casting shadows.
Refraction
Refraction is complicated to explain with the corpuscular theory. It involves the passing of air corpuscles into another medium, which increases their speed and changes their direction.
Arguments against the Corpuscular Theory:
- Bodies emitting corpuscles should lose mass, which does not happen.
- It does not explain the differences in corpuscles’ ability to reflect and refract.
- It does not explain light scattering or the white light prism.
Huygens’ Principle and the Wave Theory of Light
Christiaan Huygens stated that light is a wave that propagates longitudinally, like mechanical waves. He argued that:
- Bodies with varying mass do not emit light when they collide.
- The wave theory explains why nothing happens when light rays intersect (no darkness is produced).
- Rectilinear propagation is explicable by Huygens’ principle.
- Light reflection and refraction are typical wave phenomena.
Arguments against the Wave Theory:
- It does not explain light propagation in a vacuum.
- No interference or diffraction phenomena were observed in light at the time.
- Newton’s prestige favored the corpuscular theory.
De Broglie’s Wave-Particle Duality
De Broglie’s theory explains the dual behavior of light, stating that it can behave as both a wave and a particle.
Refraction and Reflection of Light
When a medium has a higher refractive index (n) than another, light travels slower in that medium, and the wavelength becomes smaller.
Reflection of Light
Reflected light changes direction but maintains the same speed. The angle of incidence (i) equals the angle of reflection (r).
Snell’s Law
When a ray or beam is incident normal to a surface, the reflected ray lies in the same plane, and the angle of incidence equals the angle of reflection.
Reflection on a regular polished surface is called specular reflection, while reflection on an irregular or uneven surface is called diffuse reflection.
Refraction of Light
Refraction is the change in the propagation direction of a light beam as it passes obliquely from one medium to another.
If light passes from a medium with a lower refractive index to one with a higher refractive index (+ refracting), the refracted ray bends towards the normal, and vice versa.
The critical angle is the angle of incidence at which the refracted ray emerges tangent to the surface separating the two media. If the angle of incidence is greater than the critical angle, total internal reflection occurs.
Prisms and Light Scattering
A prism is a transparent optical medium limited by two flat, non-parallel surfaces. The angle between these surfaces is called the prism angle.
Light scattering is the phenomenon where white light is decomposed into its constituent colors, forming a rainbow. This happens because different wavelengths of light are refracted at different angles. Red light deviates less than violet light because it has a longer wavelength.
Light propagates at different speeds in different material media, just like in a vacuum. This range of wavelengths forms the visible spectrum.
Light Diffraction
Light diffraction occurs when a wave undergoes a change in direction as it encounters a crack or obstacle. This allows the wave to propagate beyond the obstacle.
Optical Images and Lenses
Images can be real or virtual, small or large, and inverted or upright.
Spherical Diopters
Convex diopters have a positive radius of curvature, while concave diopters have a negative radius.
Focus of a Diopter
The focus of a diopter is the point where parallel rays converge after passing through the diopter.
Object Focus of a Diopter
The object focus of a diopter is the point from which rays emerge parallel to the optical axis after passing through the diopter.
Plane Mirrors
A plane mirror is a flat, smooth surface that can reflect light.
Spherical Mirrors
Spherical mirrors can be convex or concave. If the reflecting surface is on the external side, it is a convex mirror, and vice versa.
Thin Lenses
Thin lenses have a thickness that is small compared to other quantities, like the radius of curvature. They can be convergent or divergent. Convergent lenses are thicker at the center, while divergent lenses are thinner at the center.
Power of a Lens (Diopter)
The power of a lens is measured in diopters. A lens with a focal length of 1 meter has a power of 1 diopter.
Object Focus of a Lens
The object focus of a lens is the point where rays emerge parallel to the lens axis after passing through the lens.
Image Focus of a Lens
The image focus of a lens is the point where rays converge after passing through the lens.
Focal Lengths and Distances
- Focal-object distance: The distance between the object focus and the lens.
- Focal-image distance: The distance between the image focus and the lens.
- Lens-image distance: The distance between the image and the lens.
Lens Aberrations
Lens aberrations are defects that occur in images due to the failure to satisfy the thin lens approximation.
- Chromatic aberration: Occurs because the refractive index of a lens varies with the wavelength of light. It can be corrected by combining a diverging lens with a converging lens.
- Spherical aberration: Occurs when parallel rays do not intersect at a single point after passing through a lens. It can be reduced by using a diaphragm to block rays that are far from the optical axis.
Eye Defects
Light enters the eye through the cornea. The iris regulates the amount of light entering the eye through the pupil. The cornea-lens system focuses light on the retina, which is formed by cones and rods.
The lens is a convex lens that enables image formation on the retina with the help of ciliary muscles. The nearest point at which the lens can focus light is called the near point, which is about 25cm. The farthest point is called the far point.
Myopia (Nearsightedness)
In myopia, the retina does not focus parallel rays from distant objects correctly. The image forms in front of the retina, causing distant objects to appear blurry. This can be caused by a cornea that is too curved or an eye that is too long. Diverging lenses are used to correct myopia.
Hypermetropia (Farsightedness)
Hypermetropia is the opposite of myopia. Light rays from near objects focus behind the retina, causing near objects to appear blurry. Converging lenses are used to correct hypermetropia.
Presbyopia
Presbyopia is an age-related condition that causes eyestrain and loss of accommodation. Converging lenses are used to correct presbyopia.
Astigmatism
Astigmatism is a condition where the cornea is not perfectly spherical, causing it to focus horizontal and vertical lines differently. Cylindrical lenses are used to correct astigmatism.
Cataracts
Cataracts occur when the lens loses its transparency.