P3 Physics

X-rays                                                                                                                       X-rays are used in hospitals:                                                                         

• To make images and CT scans.                                                          
• To destroy tumors at or near the body surface.

  X-rays can damage living tissue when they pass through it.                                   X-rays are absorbed more by bones and teeth than by soft tissue.

X-rays can cause cancer, but can also be used to treat it.

Ultrasound                                                                                                  

•  Ultrasound waves are sound waves of frequency above 20 000Hz.
• Ultra sound can be used for diagnosis and treatment.
• Ultrasound waves are partly reflected at a boundary between two different types of bodily tissue.
• An ultrasound scan is non ionising so it is safer than an X-ray.

Refractive Index

Refractive Index,n, is a measure of how much a substance can refract a light ray.

Refractive Index=Sin of the Incident Angle divided by Sin of the Refractive Angle.

Incident Angle and Refractive Angle  are measured between the ray and the normal.

A ray of light traveling along the normal is not refracted.

The Endoscope

The critical angle is the angle of incidence of a light ray in a transparent substance which produces refraction along the boundary.

Refractive Index= 1 divided by sin of the Critical Angle.

Total internal reflection occurs when the angle of incidence of a light ray in a transparent substance is greater than the critical angle.

An endoscope is used to see inside the body directly.

Total Internal Reflection only takes place for a ray traveling from a more dense to less dense material eg. from glass into air.

Lenses

A converging lens focuses parallel rays to a point called the principal focus.

A diverging lens makes parallel rays spread out as if they came from 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.

A virtual image is formed by a diverging lens, and by a converging lens if the object is nearer to the lens that the principal focus.

Magnification= Image Height divided by Object Height.

Using Lenses

A ray diagram can be drawn to find the position and nature of an image formed by a lens.

When an object is places between a converging lens and F, the image formed is virtual, upright, magnified and on the same side of the lens as the object.

A camera contains a converging lens that is used to form a real image of an object.

A magnifying glass is a converging lens that is used to form a virtual image of an object.

The Eye

Light is focused on to the retina by the cornea and the eye lens, which is a variable focus lens.

The normal human eye has a range of vision from 25cm to infinity.

Power(D)= 1 divided by Focal Length(m)

More about the Eye

A short sighted eye is an eye that can only see near objects clearly. We use a diverging (concave) lens to correct it.

A long sighted eye is an eye that can only see distant objects clearly. We use a converging (convex) lens to correct it.

The higher the refractive index of the glass used to make a corrective lens, the flatter and thinner the lens can be.

The difference between the structure of an eye and a camera:

The camera has a lens of fixed shape but variable position. The eye has a lens of variable shape but fixed position.

Moments

The moment of a force is a measure of the turning effect of the force on an object.

Moment(Nm)= Force(N) times Perpendicular Distance(m).

To increase the moment of a force F, increase F or increase D.

Centre of Mass

The centre if mass of an object is that point where its mass can be thought to be concentrated.

When a suspended object is in equilibrium, its centre of mass is directly beneath the point of suspension.

The centre of mass of a symmetrical object is along the axis of symmetry.

Moments in Balance

For an object in equilibrium, the sum of the anti-clockwise moments about any point = the sum of the clockwise moments about that point.

To calculate the force needed to stop an object turning we use the equation above.

If all the distances are given in cm the unit of the moment will be newton-centimetre.

Stability