Have you ever wonder how transition lens work?

These photochromic lenses darken when exposed to bright light and will gradually return to their clear state in the absence of radiation when one walks indoors. They are good substitutes of sunglasses as they can reduce up to 80% of the transmitted light.  The change in colour in response to light can be explained by redox reactions.


During the manufacturing process, silver chloride (AgCl) and copper(I) chloride (CuCl) crystrals are added to the molten glass in its molten state and these crystals are evenly distributed in the glass.

Silver chloride is susceptible to undergo redox reaction in the presence of light as described below:

Cl ———–> Cl + e

Ag+ + e ———> Ag

The chloride ions are oxidized to produce chlorine atoms and an electron. The electron is then transferred to silver ions to produce silver atoms. These atoms cluster together and block the transmittance of light, causing the lenses to darken. This process is reversible which allows the lenses to become transparent again.

The presence of copper (I) chloride reverses the darkening process. When the lenses are removed from the light, the following reactions occur:

Cl + Cu+ ——> Cu2++ Cl

The chlorine atoms formed by the exposure to light are reduced by the copper ions, preventing their escape as gaseous atoms. The copper (+1) ion is oxidized to produce copper (+2) ions, which then reacts with the silver atoms:

Cu2+ + Ag ——> Cu++ Ag+

The effect of these reactions is that the lenses become transparent again because the silver and chloride atoms are converted to their original oxidized and reduced states.

This is just one example of Chemistry in our daily life. Find out more about how Chemistry applies to everyday occurrences in Ms Sim’s A Level Chemistry tuition class.

As parents who want the best for our children and recognising the leap in standards between O level Chemistry and A level Chemistry, parents need to decide whether or not their child needs Chemistry tuition in JC. Here are three considerations that will help parents make an informed decision on whether or not to send your child for Chemistry tuition!


1) Pure Chemistry Or Combined Science At O Levels

If your child has taken Combined Science at O Levels, it could be tough for your child to cope with A Level Chemistry due to the gulf in standards on the content was covered in Combined Science as compared to A Level Chemistry. Hence if your child has taken Combined Science, it is very likely that your child would need additional help in A Level Chemistry.


2) Grade Attained For O Level Pure Chemistry

Due to the huge leap in difficulty between O and A Level Chemistry, if your child has scored an A2 or below for O Levels, parents are strongly advised to send their child for Chemistry tuition.


3) Looking At Percentile Rather Than Grade

Parents often look at grades to determine how good or bad their child is doing. The truth is that it is the percentile, and not the grade, that predicts how well a student will perform in the A Level examinations. For example, if a particular junior college has on average 50% of its students scoring A’s in their A Level Chemistry, and if your child has scored B (but is ranked at the 55th percentile), it is likely that he/she will score an A in Chemistry as he/she is above the 50th percentile. On the contrary, if your child scored an A but is in the 35th percentile, it is unlikely that he/she will score an A in the A Level Chemistry examination as he/she is below the 50th percentile. To sum, a student’s final grade is relative to the cohort performance. Hence it is the percentile score that matters!

Click on the button below to enrol for Ms Sim’s A Level Chemistry tuition.