GEOMETRICAL OPTICS

Optics

• Optics is the study of light and how it behaves.
• Light is essential for human survival and understanding the universe.
• Optics remain a key diagnostic technique in biomedical science.

Optics in Medicine

• Optics improves illumination, magnification, and access to body cavities for diagnosis.
• Optics interacts with biological cells, tissues, and organs for probing, imaging and inducing changes for therapeutic purposes.
• Novel devices, software and methods use visible and near-infrared light to characterise cancer, track therapies and diagnose diseases.

The Human Eye

• The human eye detects visible light frequencies.
• The pupil controls the amount of light entering.
• The retina, containing rods and cones, detects light and color.
• Rods detect light intensities; cones detect color.

Photometry

• Photometry is the science of measuring light in terms of perceived brightness.
• The human eye is not equally sensitive to all wavelengths of light.
• Photometry weights each wavelength according to how sensitive the eye is at each wavelength.
• Photometry provides information on luminous intensity, flux, luminance and illuminance.

Luminous Intensity & Luminance

• Luminous intensity is the power emitted by a light source in a particular direction, measured in candela (cd).
• Luminance is the luminous intensity emitted per unit area, measured in candela/m². It shows how bright surfaces appear.

Luminous Flux or Luminous Power (Φ)

• Luminous flux (Φ) is energy per unit time radiated within the visible wavelengths (330-780 nm).
• It's a measure of the perceived power of light.
• Luminous flux is a weighted average of radiant flux to account for varying eye sensitivity at different wavelengths.
• The SI unit of luminous flux is the lumen (lm).

Illuminance (E)

• Illuminance (E) is the total luminous flux incident per unit area or the amount of light received by an object.
• The SI unit for illuminance is the lux (lx) or lumens/m².
• 1 lux = 1 lumen/m².

Lux vs. Lumen

• Lux takes into account the area over which the luminous flux is spread.
• 100 lumens concentrated into 1m² will create 100 lux.
• The same 100 lumens spread across 100m² will give 1 lux.

Candela, Lumen, Lux

• Candela measures luminous intensity in a specific direction.
• Lumen measures total luminous flux in all directions.
• Lux measures illuminance, how much light falls on a surface.

Measurement of Light

• Measurements include luminous flux, luminous intensity, illuminance and luminance.
• The use of unit candela, lumens, and lux in relation to specific types of light measurements.

Effect of Distance on Luminous Intensity

• The distance from the light source does not affect the amount of candela.

Effect of Distance on Illuminance

• Illuminance (I) is inversely proportional to the square of the distance (r²) from the light source (I ∝ 1/r²).
• The illuminance of a surface is 1/4 of the illuminance of a surface at twice the distance from the source.

Effect of Distance on Illuminance (Second)

• The amount of light output (lumen) from the source remains constant regardless of distance.
• A source twice the distance, needs four times the brightness to provide the same illuminance to the surface as the closer source.

The Effect of Distance

• Lumen is independent of distance, as it measures total light output from a source in all directions.

Fundamentals of Optics

• Reflection is the bouncing back of light from a surface.
• Refraction is the bending of light as it passes from one medium to another.
• Imaging is the formation of a representation of an object by light rays.
• Diffraction is the spreading of light as it encounters an obstacle.
• Interference is the superposition of light waves resulting in a new wave pattern.
• Polarisation is the orientation of the electric field vector of a light wave in a specific direction.

Reflection

• Real images are formed where light rays converge (a real focal point).
• Real images can be projected on a screen; they are always inverted.
• Virtual images are formed where light rays appear to come from, but don't actually reach that location.
• Virtual images cannot be projected onto a screen; they appear upright.

Virtual Images in Plane Mirrors

• Virtual images are formed behind mirrors.
• They are upright and the same size as the object.
• The distance to the image is equal to the distance of the object from the mirror.

Reflection (Second)

• Mirrors reflect light with high efficiency when the surface is very smooth to the wavelength of light being reflected.

Curved Mirrors

• Parabolic mirrors reflect parallel light rays to a single focal point.
• Parabolic mirrors are precisely shaped to focus parallel rays into a single point, which gives a sharp and focused image.
• Spherical mirrors can suffer from spherical aberration, where parallel rays reflect in a diffused pattern rather than focusing at a single point.

Properties of a Mirror

• Focal length (f) is the distance from the mirror to the focal point.
• Focal point (F) is where parallel rays converge after reflection.
• Radius of curvature (R) is the distance from the mirror surface's vertex to its center of curvature.

Parabolic vs. Spherical Mirrors

• Parabolic mirrors reflect all incoming parallel rays to a single focal point, resulting in a sharp, focused image.
• Spherical mirrors have a constant curvature, resulting in spherical aberration: a blurry image.

Parabolic Mirror

• When parallel light rays hit a parabolic mirror, they all reflect to a single point, the focal point.

Spherical Mirror

• Spherical mirrors are either concave or convex, depending on which side of the sphere is reflective.
• Image formation depends on the mirror's size relative to its radius of curvature.

Types of Curved Mirrors

• Concave mirrors converge light; convex mirrors diverge light.
• Concave mirrors can produce magnified, inverted, real images.
• Convex mirrors can produce reduced, upright, virtual images.

Concave vs. Convex Mirror

• Concave mirrors are curved inward, while convex mirrors are curved outward.
• Concave mirrors typically produce larger, inverted, real images when the object is far away.
• Convex mirrors always produce smaller, upright, virtual images.

Object Position and Image Formation (Concave Mirror)

• Focal Point (f) - Object distance (u) - Image distance (v)

Lateral Magnification

• Lateral magnification (m) is the ratio of image height to object height (h'/h).
• Positive magnification means the image is upright; negative magnification means the image is inverted.

Refraction

• Light bends when it passes from one medium to another with a different optical density.
• Speed of light is slower in denser mediums.

Refraction at Plane Surface

• Light bends at the interface between substances with different refractive indices.
• The amount of bending depends on the difference in the refractive indices of the two substances.

Snell's Law

• Snell's law relates the angles of incidence and refraction to the refractive indices of the two substances.
• Light bends towards the normal when going from a less dense to a denser medium.

Refraction - Human Eye

• Refraction in the eye focuses light onto the retina.
• The cornea and lens refract light from objects to create a sharp image on the retina.
• Water has a higher optical density than air, so light is refracted less in water, resulting in blurry vision without corrective lenses.

Refraction - Human Eye (Second)

• Light refracts slightly less in the eye when viewing underwater.
• This requires additional refraction (using corrective lenses or goggles).

Refraction from distance objects

Refraction of light from close objects

• Light rays from close objects diverge - to focus the image properly the eye increases the curvature of the lens

Reflection vs. Refraction

• Reflection is the bouncing of light off a surface.
• Refraction is the bending of light as it passes from one medium to another.

Lenses

• Lenses are transparent objects that bend light to form images.
• Spherical lenses have curved surfaces; planar (flat) lenses have a flat surface.

Types of Lenses

• Converging lenses (convex) cause parallel light rays to converge.
• Diverging lenses (concave) cause parallel light rays to diverge.

Lenses and Light

• Converging lenses focus rays at a point.
• Diverging lenses spread rays apart.

Positive or Converging Lens

• Positive/converging lenses focus collimated light rays to a single point on the axis, a distance behind the lens- called the focal point.

Positive or Converging Lens (Second)

• Biconvex lenses are converging lenses.

Negative or Diverging Lens

• Biconcave lenses are diverging lenses; they cause parallel light rays to spread out.

Concave vs. Convex Lens

• Concave lenses diverge light rays, while convex lenses converge them.
• Concave lenses are used to correct nearsightedness.
• Convex lenses are used to correct farsightedness.

Lens Maker's Equation

• The lens maker's equation relates focal length to refractive index and radii of curvature of lens surfaces.
• It's used in designing lenses with specific focal lengths and magnifications.

Imaging Properties

• A positive lens focuses light rays that travel parallel to the axis to a focal point, a specific distance from the lens.
• A point source at the focal point is converted to a collimated beam by the lens.
• The ability of lenses to create images.

Lens Formula

• The lens formula describes relationships between object distance, image distance and focal length.
• This formula connects the object distance (u), image distance (v) and focal length (f) of a lens mathematically.

Converging Lenses

• Image formation with converging lenses depends on the object's position relative to the focal points (F) and center of curvature (2F).
• This gives rise to real, inverted images, or virtual, upright images depending on the position of the object relative to the focal point of the lens.

Diverging Lenses

• Images produced by diverging lenses are always virtual, upright (not inverted), and reduced. (smaller than the object.)

How Vision Works

• Light rays are reflected from an object, enter the eye through the cornea and are refracted by the cornea, lens, and vitreous fluid.
• The lenses of the eye focus the rays to create an upside-down image on the retina.

Eye as a Camera

• The eye, similar to a camera, uses a diaphragm (pupil) to control light, a lens to focus light, and a sensor (retina) to capture the image.
• The different parts of the eye help refract light to produce a focused image on the back of the eye (retina).

Optical Lenses and Devices

• Optical lenses and devices are used in diagnosis, treatment, and various medical procedures using light and its interaction with living tissues.
• Use of lenses as prosthetics to correct visual impairments such as myopia, hyperopia, and astigmatism.
• Medical uses/prosthetics, image formation, illumination, spectroscopy, microscopy endoscopy and laser delivery.

Common Vision Defects

• Nearsightedness (myopia) : occurs when the eye is too long, causing light to focus in front of the retina.
• Farsightedness (hyperopia): occurs when the eye is too short, causing light to focus behind the retina.
• Astigmatism: the eye's cornea or lens isn't perfectly spherical. The light rays are refracted unevenly, creating blurry images,

Correction of Common Vision Defects

• Myopia is corrected with diverging lenses (thinner in the center, like a concave lens).
• Hyperopia is corrected with converging lenses (thicker in the center, like a convex lens).
• Astigmatism is corrected with cylindrical lenses, which are curved in one direction.

Myopia

• Myopia occurs when the eye is too long, resulting in light focusing in front of the retina instead of on it.
• Myopia is corrected with concave lenses.

Hyperopia

• Hyperopia is a condition where the eye is too short, causing the light to be focused behind the retina, so the image is blurry.
• It is corrected with convex lenses.

Astigmatism

• Astigmatism occurs when the cornea or lens is not perfectly spherical, resulting in uneven or multiple focal points, leading to distorted images.
• It is corrected with cylindrical lenses.

Optical Fibers (Fiber Optics)

• Optical fibers are thin, flexible tubes of glass or plastic used to transmit light.
• Light travels along the fiber by total internal reflection. This means the light reflects off the walls of the fiber without escaping.

Optical Fibers (Second)

• Optical fibers are used in medical imaging, industrial imaging, communications, and other applications.

Endoscope

• An endoscope is a flexible tube with a light source and a camera that allows doctors to view the inside of the body without major surgery.
• It is used in various diagnostic and surgical procedures in the digestive, respiratory, urinary, or reproductive systems.

Endoscope (Second)

• Components of an endoscope, including the insertion, control, and connector sections.
• Uses for endoscopes in different regions of the body.

Endoscope (Third)

• The light source is generally located at one end, illuminating the area of interest.
• The reflected light is then gathered and sent to the viewing lens at the other end, allowing the user to view the internal structures.
• Various types of endoscopes, such as those for the digestive, respiratory, urinary, and reproductive systems.

Endoscopy Work

• Different types of endoscopies, such as upper and lower endoscopy, to view different areas of the body.
• The use of light and fiber optics to illuminate and view internal cavities.

Use of Endoscopes

• Endoscopes are used in various medical procedures, including arthroscopy, bronchoscopy, endoscope biopsy, gastroscopy, and laparoscopy.

Endoscopic Ultrasound (EUS)

• EUS uses an ultrasound probe attached to an endoscope to produce detailed images of internal organs, such as the pancreas, stomach, bowel, lymph nodes, and blood vessels.

Microscope

• Microscopes are instruments used to view objects that are too small to be seen with the naked eye.
• Optical microscopes use lenses to magnify images.

Electron Microscopy (SEM & TEM)

• Electron microscopes use a beam of electrons instead of light to produce higher-resolution images.
• Scanning electron microscopy (SEM) images the surface of a sample.
• Transmission electron microscopy (TEM) images the internal structures of a very thin sample by passing electrons through it.

Compound Microscope

• A compound microscope uses a compound lens system that combines multiple lenses to magnify the image.
• The eyepiece and objective lenses contribute to the higher magnification; a condenser at the bottom illuminates the sample.

Types of Microscopes

• Optical microscopes, electron microscopes (TEM and SEM), and scanning probe microscopes.
• Key differences, including date of invention, principle of operation, sample preparation and resolution.