Similarities of astronomical telescope and a compound microscope

• consists of two convex lenses.
• The first image is real,inverted and acts as the object for the eyepiece.
• The eyepiece acts as a magnifying lens.(For an astronomical telescope in normal adjustment,the first image is at the common focal plane of the objective lens and the eyepiece.)
• The final image is virtual,invert and magnified.

 

Differences
Type of lens  

• Compound microscope:two high powered convex lenses
• Astronomical telescope:a low powered convex lens and a high powerted convex lens

First image

• Compound microscope:first image is magnified
• Astronomical telescope:first image is dimnished

Condition of the eye when using the instrument

• Compound microscope:ciliary muscles are at ease and the eye is relaxed
• Astronomical telescope:ciliary muscles are contracted.Eye is strained.   

  Ray Diagram

1. We can use ray diagram to determine the position and characteristic of images formed by lenses for any position of the object.
2. Ray diagram can be complete using any two out of the three rays of light.

  Rules for ray diagram:

convex lens

•  a ray of light which passes through the optical centre,O of the lens is undeviated.
• a ray  of light parallel to the principal axis is refracted and passes through the principal focus,F.
• a ray of light which passes through the principal focus,F is refracted parallel to principal axis.

concave lens

• a ray of light which passes through the optical centre,O is undeviated.
• a ray of light parallel to the principal axis is refracted and appears to come from the principal focus,F on same side of lens.
• a ray of light which travel towards the principal focus,F on the other of the lens is refracted parallel to the principal axis.

          *any two of the three rays point 1,2 and 3 are sufficient      to determine the position and characteristics of the image*

  Differences between convex and concave lenses

Convex lenses                                        

• Convex lenses are also known as converging lenses or positive lenses.    

• Convex lenses are thicker in the centre than at the adge.  

• Convex lenses refract incident rays of light,which are parallel to the principal   axis. This point is called the real principal focus.

Concave lenses

• Concave lenses are also known as diverging lenses or negative lenses.
• Concave lenses are thinner in the centre than at edge.
• Concave lenses refract incident rays of light,which are parallel to the principal axis,so that they appear to diverge from a point located on the incident side of the principal axis.This point is called the virtual principal focus.  
  

                                   

Total Internal Reflection

• total internal reflection is the total reflection of a beam of light at the   boundary of two mediums,when the angle of incidence in the optically denser medium exceeds a specific critical angle.
• the boundary between the two mediums acts like a perfect plane mirror where total internal reflection occurs.
• the critical angle is the angle of incidence in the optically denser medium for which the angle of reflection in the less dense medium is 90'.
• the conditions required for the occurence of total internal reflection are:

1. the light ray must travel from an optically denser medium to a less dense medium.
2. the angle of incidence must be greater than the critical angle,that is i>c

• example:it will occur when passing from glass to air.

Application-application of total Internal Reflection 

• optical fibres -> used in endoscopes and telecommunications.
• rain sensors -> to control automatic windscreen.
• prismatic binoculars -> to get a very clear image.
• gonioscopy -> to view the anatomical angle formed between the eye's cornea   and iris.
• fingerprinting devices -> to record an image of a person's fingerprint without the use of ink.

Causes of Reflection

• When a ray of light is reflected from a smooth surface, the angle of incidence is equal to the angle of reflection (i = r).

• The incident ray, the reflected ray and the normal all lie in the same plane.

Phenomenon of refraction