X-rays are part of the electromagnetic spectrum, they have a high frequency and a very short wavelength. They absorb metal and bone and are transmitted by tissue.CT scanners use x-rays to produce digital images of a cross section through the body.
X-rays cause ionisation and can damage living tissue when they pass through it therefore workers should wear film badges and lead to protect them. X-rays can also be used to treat cancerous tumours near the bodys surface.
Sound waves higher than 20,000hz are called ultrasound waves. When a ultrasound wave meets aboundary between two different materials part of the wave is reflected.
The wave travels back through the material to a detector, the time it takes to travel back can be used to calculate how far away the boundary is. The results can be processed to give an image. Ultrasound are used in medicine such as therapy or to scan unborn babies as it is much less ionisng than xrays.
Refraction is the change of direction of light as it passes from one transparent substance into another. Refraction takes place becasue the waves change speed when they cross a boundary which causes a change in direction unless the waves are travelling along the normal.
A light ray will refract when it crosses from air into glass and is refracted towards the normal. The refractive index of a substance is a measure of how much the substance can refract a light ray, it is given by this equation:
Refractive index = sine of the angle of incidence ~ sine of the angle of refraction
If the angle of incidence is increased, the angle of refraction increases until the refracted ray appears along the boundary. This angle of incidence is called the critical angle. If the angle of incidence is increased beyond the critical angle, the light ray undergoes total internal refraction where the angle of reflection is equal to the angle of incidence.
The Endoscope and Lenses:
An endoscope is a device used to look inside a patients body without cutting them open or using keyhole surgery. The endoscope contains bundles of optical fibres which are very thin, flexible glass fibres. Visible light can be sent along the fibres by total internal reflection.
Parallel rays of light pass through a converging lense (convex) are refracted so that they converge to a point called the principal focus. A real image is formed by a converging lens if the object is further away than the principal focus and a virtual image is formed if the object is nearer to the lens than the principal focus.
Parallel rays of light pass through a diverging lens (concave) are refracted so that they diverge away from a point called the principal focus. A virtual image is formed by a diverging lens.
The distance between the centre of the lens and the principal focus is called the focal length. There is a principal focus on either side of the lens.
A ray diagram can be drawn to find out the position and nature of an image formed by a lens. The line through the centre of the lens and at right angles to it is called the principal axis. Ray diagrams use three construction rays from a single point on the object to locate the corresponding point on the image:
- a ray parallel to the principal axis is refracted through the principal focus
- a ray through the centre of the lens travels straight on without refraction
- a ray through the principal focus is refracted parallel to the principal axis
Light enters the eye through the cornea which focuses the light onto the retina. The iris ajusts the size of the pupil to control the amount of light entering the eye. The ciliary muscles alter the thickness of the lens to control the focusing of the eye. They are attached to the lens by the suspensory ligaments.
A person with short sight can see objects that are close up but distant objects are blurred because the uncorrected image is formed infront of the retina. Short sight is caused by the eye being too long or the eye lens being too powerful. Short sight may be corrected using a diverging lens.
A person with long sight can see distant objects cleary but close up objects are blurred because the uncorrected image is blurred behind the retina. Long sight is caused by the eyeball being too short or the eye lens being too weak. Long sight may be corrected using a converging lens.