Bono explained why he wears sunglasses seemingly all the time whilst appearing on a British talk show.

Bono spotted in London on Thursday (16th Oct.).

Read More: Bono Apologies Over Unwanted Downloads Of U2's New Album.

The U2 frontman was on The Graham Norton Show on Friday (17th October), alongside his fellow band members and actors Robert Downey Jr., Robert Duvall and Stephen Fry. Bono was asked by Norton about his sunglasses when Norton was showing photos of the group in the early stage of their career, when Bono was notably without his glasses.

Norton asked "Do you ever take them (the glasses) off?" To which Bono replied "I have glaucoma. For the last 20 years." Bono further explained his vision is slightly impaired as he sees some objects and lights accompanied by rings and cloudiness, both of which are typical signs of Glaucoma. 

Bono in Dublin in September 2014.

Glaucoma is usually caused by the build up of pressure in the eye as the fluid (called aqueous humour) contained in the eye is unable to drain properly. According to the NHS, this build up of pressure can cause damage to the optic nerve and, if left untreated, can cause permanent visual impairment. Fortunately, Bono explained he is receiving treatment and it is a manageable condition. 

Downey was supportive and said "I just think they look cool. I think Bono looks cool in them." Bono and Downey later swapped glasses - Bono donning clear specs - and Bono joked about how his fans would now feel sorry for him. He joked "You're not going to get this out of your head now, and you will be saying, 'Ah, poor old blind Bono.'"

Read More: Neil Young Recalls The Time Bono Tried To Give Him Song Writing 'Advice'.

Watch Bono On The Graham Norton Show: